Identifying in-range fuel pressure sensor error

ABSTRACT

Methods and systems are provided for diagnosing an in-range error of a pressure sensor arranged downstream of a lift pump in a fuel system of a vehicle. In one example, a method may include performing feedback control of the lift pump based on output of the pressure sensor, monitoring the pressure sensor output for flattening during the application of the voltage pulses, and adjusting operation of the fuel system depending on whether the pressure sensor output flattens for at least a threshold duration, which is indicative of an in-range error. The method may further include dynamically learning a setpoint pressure of a pressure relief valve of the fuel system and a fuel vapor pressure within the fuel system by monitoring pressure sensor output while adjusting the duty cycle of voltage pulses applied to the lift pump.

FIELD

The present description relates generally to methods for diagnosing anin-range error of a pressure sensor arranged downstream of a fuel liftpump in an internal combustion engine and adjusting fuel systemoperation in response to the diagnosis.

BACKGROUND/SUMMARY

Internal combustion engines may include a fuel system with a fuel railfor distributing fuel to one or more fuel injectors, which may be directinjectors and/or port injectors. In a fuel system operating with directinjectors, a fuel lift pump supplies fuel to a high pressure fuel pumpthat in turn provides fuel at a high injection pressure to a fuel rail.The fuel rail is coupled to direct injectors that inject the fueldirectly into combustion chambers of the engine. In a fuel systemoperating with port fuel injection, a fuel lift pump supplies fuel at alower injection pressure to a fuel rail. The fuel rail is coupled to theport injectors, which inject the fuel into the engine intake upstream ofintake ports of the combustion chambers. In a port fuel direct injectionfuel system, both port injection and direct injection of fuel isperformed.

Regardless of the fuel system type, the fuel lift pump can be controlledto output fuel at a substantially constant delivery pressure during whatis referred to herein as continuous pump operation or operation in thecontinuous mode, via application of voltage at a duty cycle of 100% witha voltage level corresponding to the desired constant delivery pressure.When fuel flow demand changes, the voltage level may be adjusted to adifferent level and held constant or substantially constant at thedifferent voltage level (at a duty cycle of 100%), resulting in adifferent substantially constant lift pump speed and delivery pressure.In contrast, the fuel lift pump can also be controlled to outputintermittent pulses of relatively high pressure in what is referred toherein as pulsed pump operation or operation in the pulsed mode, inwhich the duty cycle of the voltage applied to the lift pump is lessthan 100%. During pulsed pump operation, the level of the voltageapplied to the lift pump may alternate between a first, higher level anda second, lower level, where the second, lower level is very low (e.g.,slightly above 0 V). During application of the first, higher level ofvoltage to the lift pump, the speed of the lift pump is high and thusthe delivery pressure of the lift pump is high, whereas duringapplication of the second, lower level of voltage to the lift pump, pumpspeed of the lift pump is very low (e.g., at a level slightly abovezero, as it may be desirable to maintain supply of voltage to the liftpump rather than intermittently provide zero voltage) and the deliverypressure of the lift pump is very low. As a result, the deliverypressure of the lift pump over time during pulsed mode operationresembles a sawtooth wave, where the duration of time between a troughof the wave and an adjacent peak of the wave following the trough isproportional to a duration of application of voltage at the first,higher level, and where the duration of time between a peak of the waveand an adjacent trough of the wave following the peak is proportional toa duration of application of voltage at the second, lower level.

In contrast to continuous pump operation, pulsed pump operation, inwhich the fuel lift pump is energized only during the duration of eachpulse, is more energy efficient. Further, when pulsed pump operation isperformed rather than continuous pump operation, durability of the fuellift pump may be extended, and maintenance costs of the fuel lift pumpmay be decreased.

When pulsed pump operation is performed, the controller of the enginemay perform either open-loop control or closed-loop control of the pump.When open-loop control is performed, voltage pulses having apredetermined pulse width (and thus, a predetermined duty cycle) may beapplied to the lift pump, and measured or inferred pressure downstreamof the fuel lift pump (referred to herein as the delivery pressure ofthe lift pump) does not influence the control. In contrast, whenclosed-loop control is performed, the delivery pressure is fed back tothe controller and influences the duration of subsequent high voltagepulses applied to the lift pump (as well as the duration of theintervals between the high voltage pulses when a voltage slightly above0 V is applied). In examples where the delivery pressure is measured bya pressure sensor that provides feedback to the controller, degradationof the pressure sensor may shift the reading of the pressure sensor andthereby cause the delivery pressure to deviate from a desired orexpected pressure, which may in turn degrade engine operation. As oneexample, errors within the expected range of sensor output (referred toas in-range errors) are much more difficult to detect than errorsoutside of the expected range of sensor output (referred to asout-of-range errors). In-range error detection is especially criticalwhen the sensor provides feedback for closed-loop control of pulsed pumpoperation, as the error will result in incorrect adjustment of thevoltage pulses applied to the lift pump.

One approach for addressing fuel pressure sensor in-range errordetection is disclosed by Stavnheim et al. in U.S. Pat. No. 6,526,948B1, which is concerned with diagnosing fuel pressure sensors that are“stuck” in-range. Therein, a controller samples a fuel pressure sensorsignal, including pressure peaks and valleys, a number of times. Thecontroller then computes an average pressure value and compares themeasured values to the average. If a measured value is within athreshold of the average value, it indicates that the pressure sensor isstuck in-range (that is, not dynamically responding to changes in fuelpressure), and the controller logs an error code. At a certain number oflogged errors, the controller initiates a minimum fueling algorithm thatsupplies just enough fuel to allow the vehicle to be driven out ofdanger or to a service center.

However, the inventors herein have recognized potential issues with thisapproach. As one example, the method described above is limited toidentifying a degraded pressure sensor that does not respond to pressurefluctuations. However, a degraded pressure sensor may read higher orlower than the actual pressure but still respond to pressurefluctuations. Further, by providing just enough fuel for the vehicle tobe driven out of danger or to a service center upon identification ofpressure sensor degradation, desired vehicle operation may beunavailable when the pressure sensor is degraded, which may have anegative impact on driver satisfaction.

To address these issues, the inventors herein have identified methodsand systems for diagnosing in-range pressure sensor errors and adjustingfuel system operation based on the diagnosis. In one example, the issuesdescribed above may be addressed by a method of operating an engine fuelsystem which comprises, during pulsed mode operation of a lift pump,adjusting a level of voltage applied to the lift pump based on an outputsignal of a pressure sensor downstream of the lift pump and monitoringthe output signal for flattening; and, in response to a detection offlattening, indicating a pressure sensor error and operating the liftpump independent of the output signal of the pressure sensor. In thisway, errors occurring within the normal operating range of a pressuresensor arranged downstream of a fuel lift pump can be detected, and fuellift pump control can be switched from closed-loop to open-loop controlupon detection of such errors. While open-loop lift pump control may beless fuel efficient than closed-loop lift pump control, it may not havea substantial impact on drivability.

In order to assure accuracy of the control of the lift pump as well asthe in-range pressure sensor error diagnosis, the method may furtherinclude dynamically learning a setpoint pressure of a pressure reliefvalve and a fuel vapor pressure of the fuel system. This may include,during steady state engine operation with a requested delivery pressureof a fuel lift pump below a first threshold, decreasing a duty cycle ofvoltage pulses applied to a fuel lift pump until flattening of an outputsignal of a pressure sensor downstream of the lift pump is detected, andstoring the pressure at which the output signal flattened as a fuelvapor pressure of the fuel system; during steady state engine operationwith a requested delivery pressure of the fuel lift pump above a secondthreshold, increasing a duty cycle of voltage pulses applied to the liftpump until flattening of the output signal of the pressure sensor isdetected, storing the pressure at which the output signal flattened as asetpoint pressure of a pressure relief valve; and adjusting lift pumpoperation based on the stored setpoint pressure and fuel vapor pressure.Dynamically learning the expected physical maximum and minimum values ofthe fuel system in this way may improve overall accuracy of the controlof the fuel lift pump, and in turn improve the accuracy of pressuresensor error diagnoses.

In yet another example in accordance with the present disclosure, thelift pump may be controlled via a robust closed-loop control strategy.This may include, during pulsed operation of a lift pump, turning thelift pump OFF when a sensed delivery pressure increases to a desiredpeak pressure or an ON time of the lift pump reaches a calibratedmaximum, and turning the lift pump ON when either the sensed deliverypressure decreases to a desired trough pressure or a volume of fuelingested by the engine reaches a predetermined volume. Such operationmay advantageously reduce the possibility of the lift pump becoming“stuck” at a pressure below the setpoint pressure when the lift pump isON due to the sensor reading low, or at a pressure above the fuel vaporpressure when the lift pump is OFF due to the sensor reading high.Optionally, the robust control strategy may also include calibratingsensor output after detecting that the ON time of the lift pump hasreached a calibrated maximum or the volume of fuel ingested by theengine has reached a predetermined volume, so that accurate lift pumpcontrol may be performed even when the sensor is degraded.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an example embodiment of a cylinder in aninternal combustion engine of a vehicle.

FIG. 2 schematically shows an example embodiment of a fuel system thatmay be used in the engine of FIG. 1.

FIGS. 3A-3E show graphs illustrating the sensed delivery pressure of afuel lift pump as a function of time during pulsed pump operation of thefuel lift pump.

FIG. 4 shows a flow chart illustrating a routine for diagnosing anin-range error of a pressure sensor downstream of a fuel lift pump andcontrolling operation of the fuel lift pump in response to thediagnosis.

FIG. 5A shows a flow chart illustrating a routine for closed-loopcontrol of a fuel lift pump.

FIG. 5B shows a flow chart illustrating a routine for closed-loopcontrol of a fuel lift pump in accordance with a first exemplaryfeedback control strategy, which may be performed in conjunction withthe routine of FIG. 5A.

FIG. 5C shows a flow chart illustrating a routine for closed-loopcontrol of a fuel lift pump in accordance with a second exemplaryfeedback control strategy, which may be performed in conjunction withthe routine of FIG. 5A.

FIG. 6 shows a flow chart illustrating a routine for adjusting operationof a fuel system with a controller to learn a pressure relief valvesetpoint pressure and fuel vapor pressure of the fuel system.

FIG. 7 shows a flow chart illustrating a routine for diagnosing anin-range error in the output of a pressure sensor downstream of a fuellift pump.

FIG. 8 shows a map of example plots of signals of interest duringadjustment of operation of a fuel system with a controller to learn apressure relief valve setpoint pressure and fuel vapor pressure of thefuel system, in accordance with the routine of FIG. 6.

FIG. 9 shows a map of example plots of signals of interest whendiagnosing an in-range error in the output of a pressure sensordownstream of a fuel lift pump, in accordance with the routine of FIG.7, where the error results in trough flattening of the pressure sensoroutput signal.

FIG. 10 shows a map of example plots of signals of interest whendiagnosing an in-range error in the output of a pressure sensordownstream of a fuel lift pump, in accordance with the routine of FIG.7, where the error results in peak flattening of the pressure sensoroutput signal.

FIG. 11 shows a flow chart illustrating a routine for robust closed-loopcontrol of a fuel lift pump.

FIGS. 12A-12D show maps of example plots of signals of interest duringrobust closed-loop control of a fuel lift pump, without calibration ofsensor output (FIGS. 12A and 12C), and with calibration of sensor output(FIGS. 12B and 12D). In FIGS. 12A-12B, the sensor is reading low,whereas in FIGS. 12C-12D, the sensor is reading high.

DETAILED DESCRIPTION

The following description relates to systems and methods for controllinga fuel lift pump in a fuel system of an engine, such as the engine shownin FIG. 1, as well as diagnosing an in-range error of a pressure sensorarranged downstream of the fuel lift pump and adjusting operation of thefuel system in response to the diagnosis. As shown in FIG. 2, the fuelsystem may include both port fuel injectors and direct fuel injectorsand associated fuel rails. However, the methods and systems describedherein are equally applicable to fuel systems which include portinjectors and do not include direct injectors, and fuel systems whichinclude direct injectors and do not include port injectors, as well asfuel systems including other types of fuel injectors which receivepressurized fuel from a fuel lift pump. The lift pump can be operated ina pulsed mode with closed-loop feedback control (e.g., in accordancewith the routine shown in FIGS. 5A-5C) in which pulses of voltage aresupplied to the lift pump until a desired fuel pressure is reached, asmeasured by a pressure sensor downstream of the lift pump. Further, asetpoint pressure of a pressure relief valve of the fuel system and afuel vapor pressure within the fuel system may be dynamically leaned ata controller of the engine by monitoring the sensed pressure downstreamof the lift pump while adjusting the voltage applied to the lift pump(e.g., adjusting the duty cycle of voltage pulses applied to the liftpump), in accordance with the routine shown in FIG. 6 and the map shownin FIG. 8. During the application of voltage pulses to the lift pump,the output signal of the pressure sensor downstream of the lift pump mayhave a sawtooth waveform, an example of which is shown in FIG. 3A.However, during an in-range error of the pressure sensor, the sawtoothwaveform may flatten at its peaks or troughs depending on the nature ofthe in-range error, as shown in FIGS. 3B-3C. As shown in FIG. 4, thecontroller may perform a routine in which the output signal of thepressure sensor downstream of the lift pump is monitored for flattening(e.g., in accordance with the routine shown in FIG. 7) during pulsedpump operation with closed-loop control. In response to a detection offlattening, an in-range error of the pressure sensor may be indicated,and the controller may switch from closed-loop pump operation (in whichpressure sensor feedback factors into the control of the lift pump) toopen-loop pump operation (in which pressure sensor feedback does notfactor into the control of the lift pump), in accordance with the mapsshown in FIGS. 9 and 10. Alternatively, the lift pump may be operated inaccordance with the robust closed-loop control strategy shown in theroutine of FIG. 11. This strategy may include turning the lift pump OFFwhen it has been ON for a calibrated maximum ON time, even if thepressure sensor output has not reached a desired peak pressure, andturning the lift pump ON when a volume of fuel ingested since the liftpump was OFF reaches a predetermined volume, even if the pressure sensoroutput has not reached a desired trough pressure, as shown in FIGS.12A-12D. Optionally, as shown in FIGS. 12B and 12D, pressure sensoroutput may be calibrated when it has been determined that the sensor isreading high or low, and the calibrated pressure sensor output may besubstituted for the pressure sensor output in the feedback control ofthe lift pump.

Regarding terminology used throughout this detailed description, portfuel injection may be abbreviated as PFI while direct injection may beabbreviated as DI. A high pressure pump may be abbreviated as a HP pump(alternatively, HPP) or a DI fuel pump. Similarly, a lift pump or fuellift pump may also be referred to as a low pressure pump (abbreviated asLP pump or LPP). Also, fuel rail pressure, or the value of pressure offuel within a fuel rail, may be abbreviated as FRP. The direct injectionfuel rail may also be referred to as a high pressure fuel rail, whichmay be abbreviated as HP fuel rail. For the sake of brevity, thepressure relief valve setpoint pressure will be referred to herein asthe setpoint pressure.

FIG. 1 depicts an example of a combustion chamber or cylinder ofinternal combustion engine 10, which may be included in a motor vehicle5. Engine 10 may be controlled at least partially by a control systemincluding controller 12 and by input from a vehicle operator 130 via aninput device 132. In this example, input device 132 includes anaccelerator pedal and a pedal position sensor 134 for generating aproportional pedal position signal PP. Cylinder 14 (herein also termedcombustion chamber 14) of engine 10 may include combustion chamber walls136 with piston 138 positioned therein. Piston 138 may be coupled tocrankshaft 140 so that reciprocating motion of the piston is translatedinto rotational motion of the crankshaft. Crankshaft 140 may be coupledto at least one drive wheel of the passenger vehicle via a transmissionsystem (not shown). Further, a starter motor (not shown) may be coupledto crankshaft 140 via a flywheel (not shown) to enable a startingoperation of engine 10.

Cylinder 14 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passages 142, 144, and 146 can communicatewith other cylinders of engine 10 in addition to cylinder 14. In someexamples, one or more of the intake passages may include a boostingdevice such as a turbocharger or a supercharger. For example, FIG. 1shows engine 10 configured with a turbocharger including a compressor174 arranged between intake air passages 142 and 144, and an exhaustturbine 176 arranged along exhaust passage 158. Compressor 174 may be atleast partially powered by exhaust turbine 176 via a shaft 180 where theboosting device is configured as a turbocharger. However, in otherexamples, such as where engine 10 is provided with a supercharger,exhaust turbine 176 may be optionally omitted, where compressor 174 maybe powered by mechanical input from a motor or the engine. A throttle162 including a throttle plate 164 may be provided along an intakepassage of the engine for varying the flow rate and/or pressure ofintake air provided to the engine cylinders. For example, throttle 162may be positioned downstream of compressor 174 as shown in FIG. 1, oralternatively may be provided upstream of compressor 174.

Exhaust manifold 148 can receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 14. Exhaust gas sensor 128 is showncoupled to exhaust passage 158 upstream of emission control device 178.Sensor 128 may be selected from among various suitable sensors forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), aNOx, HC, or CO sensor, for example. Emission control device 178 may be athree way catalyst (TWC), NOx trap, various other emission controldevices, or combinations thereof.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 14 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 14. In some examples, eachcylinder of engine 10, including cylinder 14, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 via actuator 152.Similarly, exhaust valve 156 may be controlled by controller 12 viaactuator 154. During some conditions, controller 12 may vary the signalsprovided to actuators 152 and 154 to control the opening and closing ofthe respective intake and exhaust valves. The position of intake valve150 and exhaust valve 156 may be determined by respective valve positionsensors (not shown). The valve actuators may be of the electric valveactuation type or cam actuation type, or a combination thereof. Theintake and exhaust valve timing may be controlled concurrently or any ofa possibility of variable intake cam timing, variable exhaust camtiming, dual independent variable cam timing or fixed cam timing may beused. Each cam actuation system may include one or more cams and mayutilize one or more of cam profile switching (CPS), variable cam timing(VCT), variable valve timing (VVT), and/or variable valve lift (VVL)systems that may be operated by controller 12 to vary valve operation.For example, cylinder 14 may alternatively include an intake valvecontrolled via electric valve actuation and an exhaust valve controlledvia cam actuation including CPS and/or VCT. In other examples, theintake and exhaust valves may be controlled by a common valve actuatoror actuation system, or a variable valve timing actuator or actuationsystem.

Cylinder 14 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. In one example, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen, for example, when higher octane fuels orfuels with higher latent enthalpy of vaporization are used. Thecompression ratio may also be increased if direct injection is used dueto its effect on engine knock.

In some examples, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 14 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 192 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel as may be the case with some diesel engines.

In some examples, each cylinder of engine 10 may be configured with oneor more fuel injectors for providing fuel thereto. As a non-limitingexample, cylinder 14 is shown including two fuel injectors 166 and 170.Fuel injectors 166 and 170 may be configured to deliver fuel receivedfrom fuel system 8. As elaborated below with reference to FIG. 2, fuelsystem 8 may include one or more fuel tanks, fuel pumps, and fuel rails.

Fuel injector 166 is shown coupled directly to cylinder 14 for injectingfuel directly therein in proportion to the pulse width of signal FPW-1received from controller 12 via electronic driver 168. In this manner,fuel injector 166 provides direct injection of fuel into combustioncylinder 14. While FIG. 1 shows injector 166 positioned to one side ofcylinder 14, it may alternatively be located overhead of the piston,such as near the position of spark plug 192. Such a position may improvemixing and combustion when operating the engine with an alcohol-basedfuel due to the lower volatility of some alcohol-based fuels.Alternatively, the injector may be located overhead and near the intakevalve to improve mixing. Fuel may be delivered to fuel injector 166 froma fuel tank of fuel system 8 via a lift pump and/or a high pressure fuelpump and a fuel rail. Further, the fuel tank may have a pressuretransducer providing a signal to controller 12.

Fuel injector 170 is shown arranged in intake air passage 146, ratherthan in cylinder 14, in a configuration that provides port injection offuel into the intake port upstream of cylinder 14. Fuel injector 170 mayinject fuel, received from fuel system 8, in proportion to the pulsewidth of signal FPW-2 received from controller 12 via electronic driver171. Note that a single electronic driver 168 or 171 may be used forboth fuel injection systems, or multiple drivers, for example electronicdriver 168 for fuel injector 166 and electronic driver 171 for fuelinjector 170, may be used, as depicted.

In an alternate example, each of fuel injectors 166 and 170 may beconfigured as direct fuel injectors for injecting fuel directly intocylinder 14. In still another example, each of fuel injectors 166 and170 may be configured as port fuel injectors for injecting fuel upstreamof intake valve 150. In yet other examples, cylinder 14 may include onlya single fuel injector that is configured to receive different fuelsfrom the fuel systems in varying relative amounts as a fuel mixture, andis further configured to inject this fuel mixture either directly intothe cylinder as a direct fuel injector or upstream of the intake valvesas a port fuel injector. As such, it should be appreciated that the fuelsystems described herein should not be limited by the particular fuelinjector configurations described herein by way of example.

Fuel may be delivered by both injectors to the cylinder during a singlecycle of the cylinder. For example, each injector may deliver a portionof a total fuel injection that is combusted in cylinder 14. Further, thedistribution and/or relative amount of fuel delivered from each injectormay vary with operating conditions, such as engine load, knock, andexhaust temperature, such as described herein below. The port injectedfuel may be delivered during an open intake valve event, closed intakevalve event (e.g., substantially before the intake stroke), as well asduring both open and closed intake valve operation. Similarly, directlyinjected fuel may be delivered during an intake stroke, as well aspartly during a previous exhaust stroke, during the intake stroke, andpartly during the compression stroke, for example. As such, even for asingle combustion event, injected fuel may be injected at differenttimings from the port and direct injector. Furthermore, for a singlecombustion event, multiple injections of the delivered fuel may beperformed per cycle. The multiple injections may be performed during thecompression stroke, intake stroke, or any appropriate combinationthereof.

As described above, FIG. 1 shows only one cylinder of a multi-cylinderengine. As such, each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc. It will beappreciated that engine 10 may include any suitable number of cylinders,including 2, 3, 4, 5, 6, 8, 10, 12, or more cylinders. Further, each ofthese cylinders can include some or all of the various componentsdescribed and depicted by FIG. 1 with reference to cylinder 14.

Fuel injectors 166 and 170 may have different characteristics. Theseinclude differences in size, for example, one injector may have a largerinjection hole than the other. Other differences include, but are notlimited to, different spray angles, different operating temperatures,different targeting, different injection timing, different spraycharacteristics, different locations etc. Moreover, depending on thedistribution ratio of injected fuel among injectors 170 and 166,different effects may be achieved.

Controller 12 is shown in FIG. 1 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown asnon-transitory read only memory chip 110 in this particular example forstoring executable instructions, random access memory 112, keep alivememory 114, and a data bus. Controller 12 may receive various signalsfrom sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 122; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 120 (or other type)coupled to crankshaft 140; throttle position (TP) from a throttleposition sensor; and absolute manifold pressure signal (MAP) from sensor124. The MAP signal may be used to provide an indication of vacuum, orpressure, in the intake manifold. An engine speed signal, RPM, may begenerated by controller 12 from the signal PIP.

In some examples, vehicle 5 may be a hybrid vehicle with multiplesources of torque available to one or more vehicle wheels 55. In otherexamples, vehicle 5 is a conventional vehicle with only an engine, or anelectric vehicle with only electric machine(s). In the example shown,vehicle 5 includes engine 10 and an electric machine 52. Electricmachine 52 may be a motor or a motor/generator. Crankshaft 140 of engine10 and electric machine 52 are connected via a transmission 54 tovehicle wheels 55 when one or more clutches 56 are engaged. In thedepicted example, a first clutch 56 is provided between crankshaft 140and electric machine 52, and a second clutch 56 is provided betweenelectric machine 52 and transmission 54. Controller 12 may send a signalto an actuator of each clutch 56 to engage or disengage the clutch, soas to connect or disconnect crankshaft 140 from electric machine 52 andthe components connected thereto, and/or connect or disconnect electricmachine 52 from transmission 54 and the components connected thereto.Transmission 54 may be a gearbox, a planetary gear system, or anothertype of transmission. The powertrain may be configured in variousmanners including as a parallel, a series, or a series-parallel hybridvehicle.

Electric machine 52 receives electrical power from a traction battery 58to provide torque to vehicle wheels 55. Electric machine 52 may also beoperated as a generator to provide electrical power to charge battery58, for example during a braking operation.

FIG. 2 schematically depicts an exemplary embodiment of fuel system 8 ofFIG. 1. Actuators of fuel system 8 may be operated by a controller, suchas controller 12 of FIG. 1, to perform some or all of the operationsdescribed with reference to the example routines depicted in FIGS. 4-7.

Fuel system 8 can provide fuel to an engine, such as example engine 10of FIG. 1, from a fuel tank 202. In the depicted embodiment, the fuelsystem is a PFDI fuel system and thus includes a first low-pressure fuelrail 240 which dispenses fuel to one or more port injectors 242 and asecond high-pressure fuel rail 250 which dispenses fuel to one or moredirect injectors 252. In other examples, however, fuel system 8 may be aPFI or DI fuel system. By way of example, the fuel may include one ormore hydrocarbon components, and may also optionally include an alcoholcomponent. The fuel may be provided to fuel tank 202 via fuel fillingpassage 204.

A fuel lift pump (LPP) 208 in communication with fuel tank 202 may beoperated to supply fuel from fuel tank 202 to a first fuel passage 230.As shown, first fuel passage 230 has a first end coupled to the outputof the lift pump and a second end coupled to the first fuel rail, suchthat fuel pumped into the first fuel passage by the LPP may be suppliedto the first fuel rail 240 and thus to port injectors 242. In oneexample, LPP 208 may be electrically-powered and disposed at leastpartially within fuel tank 202. As shown, a check valve 209 may bepositioned downstream of an outlet of LPP 208. Check valve 209 mayenable fuel flow from LPP 208 to first fuel passage 230, while blockingfuel flow in the opposite direction from first fuel passage 230 back toLPP 208. The pressure downstream of check valve 209 may differ from thepressure downstream of LPP 208 and upstream of check valve 209;references to the pressure in the first fuel passage herein refer to thepressure in the first fuel passage downstream of check valve 209.

A pressure relief valve 211 may be included in the fuel system to bleedoff excess pressure. In the depicted example, pressure relief valve 211is arranged in a passage 231 which has a first end coupled to first fuelpassage 230 and a second end coupled to fuel tank 202, to allow fuel toflow from first fuel passage 230 back to fuel tank 202 in the event thatthe pressure of the fuel system exceeds a setpoint pressure of thepressure relief valve. The pressure relief valve may a passive valvewhich opens and closes depending on a fluid pressure it is exposed to;alternatively, the pressure relief valve may be an actively controlledvalve, and the controller may send a signal to an actuator of thepressure relief valve to open or close the valve depending on a fluidpressure such as the delivery pressure of the fuel system. The setpointpressure is the pressure at which the pressure relief valve passivelyopens (or is actively opened) to bleed pressure from the fuel system(e.g., by returning fuel to the fuel tank). The value of the setpointpressure may be fixed by the geometry of the pressure relief valve, ormay be varied by an actuator of the pressure relief valve in response toa signal from the controller.

While first fuel rail 240 is shown dispensing fuel to four portinjectors 242, it will be appreciated that first fuel rail 240 maydispense fuel to any suitable number of fuel injectors. As one example,first fuel rail 240 may dispense fuel to one of port injectors 242 foreach cylinder of the engine. In other examples, first fuel passage 230may provide fuel to port injectors 242 via two or more first fuel rails.For example, where the engine cylinders are configured in a V-typeconfiguration, the first fuel passage may lead to two first fuel rails,each of which may dispense fuel to respective port injectors.

In the depicted example, a second fuel passage 232 branches from thefirst fuel passage upstream of the first fuel rail. A first end of thesecond fuel passage is coupled to the first fuel passage upstream of thefirst fuel rail, while a second end of the second fuel passage iscoupled to the second fuel rail. A direct injection fuel pump (HPP) 228,which receives fuel pumped from the fuel tank by LPP 208, is arranged insecond fuel passage 232. In one example, HPP 228 may be amechanically-powered positive-displacement pump. HPP 228 may be incommunication with direct injectors 252 via second fuel rail 250. Fuelpumped by LPP 208 into first fuel passage 230 may be pumped from firstfuel passage 230 into second fuel passage 232 by HPP 228, and furtherpressurized by HPP pump 228, before flowing to second fuel rail 250 forinjection directly into the engine via direct injectors 252. Second fuelrail 250 may be a high pressure fuel rail; for example, fuel may bestored in second fuel rail 250 at a pressure higher than the pressure ofthe fuel stored first fuel rail 240, due to the further pressurizationof the fuel occurring at HPP 228.

The various components of fuel system 8 communicate with an enginecontrol system, such as controller 12. For example, controller 12 mayreceive signals indicating operating conditions from various sensorsassociated with fuel system 8 in addition to the sensors previouslydescribed with reference to FIG. 1. The signals may include signal fromone or more pressure sensors arranged in the fuel system such aspressure sensors 234, 235, and 236. The signals may further include asignal from a fuel level sensor 206 indicating an amount of fuel storedin fuel tank 202. Controller 12 may also receive signals indicating fuelcomposition from one or more fuel composition sensors in addition to, oralternative to, an indication of a fuel composition that is inferredbased on a signal from an exhaust gas sensor (such as sensor 128 of FIG.1). For example, an indication of fuel composition of fuel stored infuel tank 202 may be provided by fuel composition sensor 210. Fuelcomposition sensor 210 may further comprise a fuel temperature sensor.Additionally or alternatively, one or more fuel composition sensors maybe provided at any suitable location along the fuel passages between thefuel storage tank and the fuel injectors.

In the example shown in FIG. 2, the fuel system comprises a pressuresensor 236 coupled to second fuel rail 250, and one or more of apressure sensor 234 coupled to first fuel passage 230 and a pressuresensor 235 coupled to first fuel rail 240. Pressure sensor 234 may beused to determine a fuel line pressure of first fuel passage 230downstream of the lift pump, and thus the delivery pressure of the liftpump. Pressure sensor 235 may be used for measuring the pressure levelwithin first fuel rail 240. Pressure sensor 236 may be used formeasuring the pressure level in second fuel rail 250. The locations ofthe pressure sensors shown in FIG. 2 are for exemplary purposes only andare non-limiting; instead of or in addition to the depicted pressuresensors, other pressure sensors may be positioned in fuel system 8 tomeasure the pressure at different locations therein. The various sensedpressures may be communicated as signals to controller 12. In someexamples, other types of sensors may be arranged at various locations infuel system 8, and pressures within the fuel system may be inferredbased on the output of these sensors.

As used herein, the term “delivery pressure” refers to fuel pressuredownstream of the lift pump, specifically, downstream of the check valve209 in the example fuel system of FIG. 2, and upstream of any DI pump orother type of pump that may be included in the system. In an examplewhere the fuel system includes a pressure sensor in the first fuelpassage (e.g., pressure sensor 234) and does not include a pressuresensor in the first fuel rail, the delivery pressure refers to thepressure measured in the first fuel passage. In an example where thefuel system includes a pressure sensor in the first fuel rail (e.g.,pressure sensor 235) but does not include a pressure sensor in the firstfuel passage, the delivery pressure refers to the pressure in the firstfuel rail. In an example where the fuel system includes a pressuresensor in both the first fuel passage and the first fuel rail, thedelivery pressure may refer to only one of the pressure in the firstfuel passage and the pressure in the first fuel rail.

Controller 12 is configured to control the operation of each of LPP 208and HPP 228 to adjust an amount, pressure, flow rate, etc., of fueldelivered to the engine. As one example, controller 12 may vary apressure setting, a pump stroke amount, a pump duty cycle command,and/or fuel flow rate of the fuel pumps to deliver fuel to differentlocations of the fuel system. During both port injection and directinjection, LPP 208 may be controlled by controller 12 to supply fuel tofirst fuel rail 240 and/or HPP 228 based on the pressure in one or moreof the first fuel passage, the first fuel rail, and the second fuelrail. A driver electronically coupled to controller 12 may be used tosend a control signal to an actuator of LPP 208 to adjust the output(e.g., speed and/or delivery pressure) of LPP 208. During directinjection, the amount of fuel that is delivered to the direct injectorsvia HPP 228 may be adjusted by adjusting and coordinating the output ofLPP 208 and HPP 228.

Controller 12 may control LPP 208 to operate in a continuous mode or apulsed mode. Similarly, controller 12 may control HPP 228 to operate ina continuous mode or a pulsed mode. During operation of LPP 208 in thecontinuous mode, a constant non-zero voltage is applied to the lift pumpto supply fuel at a constant fuel pressure to first fuel rail 240.Continuous mode operation of HPP 228 may be carried out in a similarmanner. On the other hand, during operation of LPP 208 in the pulsedmode, the LPP may be activated (e.g., turned ON) but provided with zerovoltage or a voltage slightly greater than zero. Pulses of highervoltage may then be supplied to LPP 208. During application of eachhigher voltage pulse, the voltage supplied to the LPP is increased froma lower positive voltage (e.g., 0 V or substantially 0 V) to a higherpositive voltage (e.g., 8-12 V), held at the higher voltage for aduration (e.g., 30-300 ms), and then decreased from the higher voltageback to the lower voltage.

In accordance with a first exemplary feedback control strategy, a dutycycle of the voltage pulses is fixed. The duty cycle of the voltagepulses determines the relative duration of application of the lowervoltage and the higher voltage to the lift pump (and thus, the pulsewidth of the pulses). In such cases, a higher voltage to be supplied tothe lift pump may be selected based on the fixed duty cycle (whichdictates the duration of the higher voltage pulses). For example, LPP208 may be pulsed at 8 V when the interval between the higher voltagepulses (during which the lower voltage is supplied) is between 0 and 50milliseconds. Alternatively, when the interval between the highervoltage pulses is between 50 and 100 milliseconds, LPP 208 may be pulsedat 10 V. In another example, LPP 208 may be pulsed at 12 V when theinterval between the higher voltage pulses is between 100 and 250milliseconds.

In contrast, in a second exemplary feedback control strategy, the LPP isturned ON (e.g., operated at a high voltage) when it is sensed that adesired trough delivery pressure has been reached, and turned OFF (e.g.,operated at a voltage near 0 V) when it is sensed that a desired peakdelivery pressure has been reached.

Operating the LPP in the pulsed mode may effectively ensure lower energyconsumption by the LPP while providing a faster response time when theLPP is actuated. Further, operation in the pulsed mode may improvedurability of LPP 208. Pulsed mode operation of HPP 228 may be carriedout in a similar manner.

A Pump Electronics Module (PEM) of LPP 208 may supply electrical powerto an electric motor coupled to the LPP. In one example, a controllersuch as controller 12 of FIG. 1 reads the output of a fuel pressuresensor sensing the delivery pressure of the LPP, and issues a Fuel PumpCommand (FPC) to the PEM which varies with and is determined based onthe output of the fuel pressure sensor, among other factors. The FPC maybe encoded as a 150 Hz duty cycle, for example, which communicates theintended duty cycle of a Field Effect Transistor (FET) of the LPP to thePEM. Alternatively, the PEM may communicate the FPC via serial interfacesuch as a CAN bus or LIN bus. The PEM takes the commanded FET duty cycleand duty cycles the FET at a frequency such as 9.8 kHz. This causes aneffective voltage to be applied to the pump's brushed DC motor. Thus, ifthe vehicle voltage supply is at 12 V and the desired effective voltageto be applied to the LPP is at 6 V, the FET may be turned on for 0.00005seconds and off for 0.00005 seconds (thus, operated at a 50% dutycycle). The PEM current has a certain value; the pump motor current isgenerally a current that is on average greater than the average currentof the PEM due to the circulation of current through a diode while theFET is off. (Instantaneous PEM current is substantially equal to pumpmotor instantaneous current while the FET is on. While the FET is off,PEM instantaneous current is zero but the current through the motor'sinductance is some positive value.) The PEM sources its electricalenergy from the vehicle battery, which may be a 12 V battery, as well asthe alternator system of the vehicle. If no “current shaping” or “softstart” actions are taken, the PEM current peaks at 30 to 35 amps, forexample. However, by not instantly applying a full step voltage of fullbattery/alternator voltage, the peak value of this in-rush current maybe reduced, e.g. to the level of steady state current. For example, theeffective pump motor applied voltage may be shaped such that the in-rushcurrent spike remains less than 10 amps.

When operating LPP 208 in the pulsed mode, a sawtooth pressure patternmay be observed in the delivery pressure, as will be discussed infurther detail with reference to FIGS. 3A-3C. For example, the pulsedmode may generate a quick rise in pressure to 6.5 bar followed by a rampdown to 4.5 bar as fuel is consumed. While this change in pressure maynot be used in direct injection systems, knowledge of current pressuremay be desired in PFI systems.

In the continuous mode of operation, control of the LPP (e.g., controlof the level of voltage applied to the LPP) may be closed-loop controlbased on feedback from one or more pressure sensors (e.g., pressuresensors 234, 235, and 236) or open-loop control which is performedindependent of, and does not take into account, pressure sensorfeedback. Similarly, in the pulsed mode of operation, control of the LPP(e.g., control of the voltage level and/or duty cycle of the pulsesapplied to the LPP) may be closed-loop control based on feedback fromone or more pressure sensors (e.g., pressure sensors 234, 235, and 236)or open-loop control which is performed independent of, and does nottake into account, pressure sensor feedback. When the pulsing of LPP 208is performed independent of feedback, the LPP may be operated withslightly higher power than required. However, despite the slightlyhigher power provided to LPP 208 during pulsed mode operation withoutfeedback, the LPP may effectively consume significantly lower power inthe pulsed mode without feedback as compared to power consumption duringcontinuous mode operation of the lift pump.

FIGS. 1-2 show example configurations with relative positioning of thevarious components. If shown directly contacting each other, or directlycoupled, then such elements may be referred to as directly contacting ordirectly coupled, respectively, at least in one example. Similarly,elements shown contiguous or adjacent to one another may be contiguousor adjacent to each other, respectively, at least in one example. As anexample, components laying in face-sharing contact with each other maybe referred to as in face-sharing contact. As another example, elementspositioned apart from each other with only a space there-between and noother components may be referred to as such, in at least one example. Asyet another example, elements shown above/below one another, at oppositesides to one another, or to the left/right of one another may bereferred to as such, relative to one another. Further, as shown in thefigures, a topmost element or point of element may be referred to as a“top” of the component and a bottommost element or point of the elementmay be referred to as a “bottom” of the component, in at least oneexample. As used herein, top/bottom, upper/lower, above/below, may berelative to a vertical axis of the figures and used to describepositioning of elements of the figures relative to one another. As such,elements shown above other elements are positioned vertically above theother elements, in one example. As yet another example, shapes of theelements depicted within the figures may be referred to as having thoseshapes (e.g., such as being circular, straight, planar, curved, rounded,chamfered, angled, or the like). Further, elements shown intersectingone another may be referred to as intersecting elements or intersectingone another, in at least one example. Further still, an element shownwithin another element or shown outside of another element may bereferred as such, in one example.

FIGS. 3A-3E depict graphs illustrating the sensed and actual deliverypressures of a fuel lift pump (e.g., LPP 208 of FIG. 2) during pulsedoperation, as a function of time. FIG. 3A illustrates a waveform thatrepresents both the sensed and actual delivery pressures during pulsedoperation when a pressure sensor sensing the delivery pressure isfunctioning properly. FIG. 3B illustrates two waveforms representing thesensed and actual delivery pressures, respectively, during pulsedoperation with the first exemplary feedback control strategy, when thepressure sensor sensing the delivery pressure is degraded and readinghigh. FIG. 3C illustrates two waveforms representing the sensed andactual delivery pressures, respectively, during pulsed operation withthe first exemplary feedback control strategy, when the pressure sensorsensing the delivery pressure is degraded and reading low. FIG. 3Dillustrates two waveforms representing the sensed and actual deliverypressures, respectively, during pulsed operation with the secondexemplary feedback control strategy when the pressure sensor sensing thedelivery pressure is degraded and reading high. FIG. 3E illustrates twowaveforms representing the sensed and actual delivery pressures,respectively, during pulsed operation with the second exemplary feedbackcontrol strategy when the pressure sensor sensing the delivery pressureis degraded and reading low.

As shown in FIGS. 3A-3E, application of voltage pulses to a fuel liftpump results in delivery pressures that create a waveform with asawtooth pattern when plotted over time. In some examples, during pulsedoperation of the lift pump, in accordance with the first exemplaryfeedback control strategy, the duty cycle of the pulses applied to thelift pump (and optionally the level of the supply voltage) is selected(e.g., pre-programmed into the controller, dynamically learned at thecontroller, or determined at the controller based on engine operatingconditions) such that application of each pulse of supply voltage to thelift pump generates a quick rise in the delivery pressure until adesired peak pressure is reached. In accordance with the secondexemplary feedback control strategy, however, a predetermined highvoltage is applied to the lift pump when it is sensed that a desiredtrough delivery pressure has been reached, whereas a predetermined lowvoltage (e.g., 0 V or slightly greater than 0 V) is applied to the liftpump when it is sensed that a desired peak delivery pressure has beenreached, such that the sensed delivery pressure dictates the duration ofeach higher voltage pulse. It will be appreciated that other feedbackcontrol strategies may be used without departing from the scope of thisdisclosure.

In the examples shown in FIGS. 3A-3E, the desired peak pressure(represented by dashed line 307) was chosen to be below a setpointpressure of the pressure relief valve (represented by dashed line 302),and the desired trough pressure (represented by dashed line 305) waschosen to be above the vapor pressure of the fuel (represented by dashedline 304). The setpoint pressure and the fuel vapor pressure mayrepresent the physical maximum and minimum pressures of the fuel system,respectively. For example, as discussed above with reference to FIG. 2,the setpoint pressure is the pressure at which the pressure relief valveopens to bleed pressure from the fuel system (e.g., by returning fuel tothe fuel tank). Further, the fuel exists in thermodynamic equilibriumbetween its gaseous and liquid phases, with the fuel vapor existing witha specific pressure (e.g., vapor pressure) that depends on fuelcomposition and temperature. In the absence of additional fuel deliveryby the lift pump, as fuel is injected by the fuel injectors, thedelivery pressure decreases to the fuel vapor pressure and cannotdecrease any further. The fuel vapor pressure may vary from near zeroabsolute pressure at cold ambient temperatures to 600+ kPa absolutepressure during hot restarts. The fuel vapor pressure is the minimumpressure that can be obtained in the fuel system as long as any liquidfuel exists in the system, which is always the case in real vehicles.Undissolved air may also exist in the line which makes the pressureslightly higher than the fuel vapor pressure, but the fuel vaporpressure still sets the minimum pressure.

Powering the lift pump motor results in the delivery pressure rising,such that the delivery pressure ends up appearing to be an upward rampwhen plotted over time. When the lift pump motor is OFF and the voltageapplied to the lift pump is substantially 0 V, and the fuel is being PFIinjected out or DI pumped out of this low fuel pressure zone at aconstant rate, the delivery pressure ends up appearing to be a downwardramp when plotted over time. If the fuel consumption (via PFI injectionsor DI pumping) increases, the downward ramp steepens and vice versa.

In the example graph 300 shown in FIG. 3A, feedback control of the liftpump is working properly, and the pressure sensor sensing the deliverypressure reads accurately (e.g., it is not degraded). As the pressuresensor is reading accurately, the signal output by the pressure sensoraccurately represents the actual delivery pressure. Accordingly,waveform 306, which has a sawtooth pattern, represents both the signaloutput by the pressure sensor and the actual delivery pressure. Asshown, waveform 306 has peaks 306 a at a desired peak pressure(represented by dashed line 307), which is lower than the setpointpressure 302 of the pressure relief valve (thereby providing a marginbetween the peak pressure and the setpoint pressure). Further, waveform306 has troughs 306 b at a pressure higher than fuel vapor pressure 304.In other examples, however, the desired peak pressure may be set equalto the setpoint pressure, and/or the duty cycle of the pulses may be setsuch that the troughs of the waveform are equal to the fuel vaporpressure.

In contrast, in the example graph 320 shown in FIG. 3B, the pressuresensor is degraded and reads high (represented by waveform 309) comparedwith the actual delivery pressure (represented by waveform 308). In thisexample, the first exemplary feedback control strategy is beingperformed. Accordingly, waveform 309 has the same shape as waveform 308but is shifted upward in the graph, as the controller adjusts thevoltage pulses applied to the lift pump in response to the (higher)sensed delivery pressure. Specifically, the controller has decreased theduty cycle of the voltage pulses applied to the lift pump to a lowervalue relative to the duty cycle that would have been selected with anaccurate sensor reading. As a result, not enough voltage is supplied forthe actual delivery pressure (waveform 308) to reach the desired peakpressure 307, and the actual delivery pressure decreases relative to thedelivery pressure during nominal pressure sensor operation (e.g., asrepresented by waveform 306 of FIG. 3A). Further, in the depictedexample, the actual delivery pressure has decreased to such an extentthat after a voltage pulse is applied to the lift pump during fuelinjection by fuel injectors, the pressure decreases to the fuel vaporpressure and remains at the fuel vapor pressure for a duration (e.g.,until it begins increasing again due to application of the next voltagepulse), such that waveform 308 appears flattened at each trough. Troughflattening occurs because the actual pressure cannot fall lower than thefuel vapor pressure 304, which is the physical minimum of the system.This flattening is in contrast to the pressure characteristic of theactual delivery pressure when the sensor is functioning properly,wherein the actual delivery pressure continues to decrease until thenext voltage pulse is applied, resulting in a sharp transition of thepressure signal from negative slope to positive slope at the troughpressure, e.g., such that the pressure signal remains at its minimumvalue for less than a threshold duration. The normal, pointed troughsthat would occur for waveform 308 if the pressure could go lower thanthe fuel vapor pressure are shown by dashed lines. Similar to waveform308, waveform 309 appears flattened at each trough, but the flatteningoccurs at a measured pressure higher than the fuel vapor pressure due tothe sensor reading high.

In the example graph 330 shown in FIG. 3C, the fuel line pressure sensoris degraded and reads low (represented by waveform 311) compared withthe actual delivery pressure (represented by waveform 310). Here again,the first exemplary feedback control strategy is being performed.Accordingly, waveform 311 has the same shape as waveform 310 but isshifted downward in the graph. In this case, the controller adjusts thevoltage pulses applied to the lift pump in response to the (lower)sensed delivery pressure by increasing the duty cycle of the voltagepulses applied to the lift pump to a higher value relative to the dutycycle that would have been selected if the signal provided by thepressure sensor was accurate. As a result, the actual delivery pressure(waveform 310) increases overall relative to the delivery pressureduring nominal pressure sensor operation (e.g., as represented bywaveform 306 of FIG. 3A). Therefore, more voltage is supplied to LPP 208than is needed to reach the desired peak pressure, which is undesirableas it decreases efficiency and increases power consumption. As shown,the peaks of waveform 310 are at a pressure higher than desired peakpressure 307. Further, in the depicted example, the actual deliverypressure has increased to such an extent that as a voltage pulse isapplied to the lift pump, the pressure increases to the setpointpressure of the pressure relief valve. The voltage then remains at thesetpoint pressure for a duration (e.g., until it begins decreasing againdue to fuel injection by fuel injectors/pumping by the DI pump), suchthat waveform 310 appears flattened at each peak. This is in contrast tothe pressure characteristic of the actual delivery pressure when thesensor is functioning properly, wherein the actual delivery pressurecontinues to increase until fuel is consumed via injection of fuel intothe engine by the fuel injectors, resulting in a sharp transition of thepressure signal from positive slope to negative slope at the peakpressure, e.g., such that the pressure signal remains at its maximumvalue for less than a threshold duration. The flattening of the peakoccurs because the actual pressure cannot exceed the setpoint pressure302. The normal, unflattened peaks that would occur if the pressurecould exceed the setpoint pressure are shown by dashed lines. Similar towaveform 310, waveform 311 appears flattened at each peak, but theflattening occurs at a measured pressure lower than the setpointpressure due to the low sensor reading.

As used herein, “flattening” of the sensed delivery pressure and actualdelivery pressure refers to an event wherein the pressure waveformtransitions from a non-zero slope to a zero slope and stays at the zeroslope (e.g., remains constant) for more than a threshold duration. Forexample, the sensed pressure may transition from a negative slope tozero slope and then to a positive slope for a trough (as shown in FIG.3B) or the other way around for a peak (as shown in FIG. 3C), in eachcase remaining at the zero slope for a threshold duration. The thresholdduration may be predetermined during engine manufacturing and stored innon-transitory memory of the control system. Further, the thresholdduration may be proportional to the duty cycle of the voltage pulsesapplied to the lift pump and in particular less than the duration (pulsewidth) of each voltage pulse. Flattening of the pressure waveform mayalternatively be referred to as clipping of the waveform at the peaksand troughs, or plateauing of the waveform at its maximum and minimumvalues.

Whereas the example graphs shown in FIGS. 3B-3C pertained to pulsedoperation of the LPP in accordance with the first exemplary feedbackcontrol strategy, the example graphs shown in FIGS. 3D-3E pertain topulsed operation of the LPP in accordance with the second exemplaryfeedback control strategy. In example graph 340 shown in FIG. 3D, thepressure sensor is degraded and reads high (represented by waveform 313)compared with the actual delivery pressure (represented by waveform312). In this example, the second exemplary feedback control strategy isbeing performed. At the beginning of the plot, the delivery pressure isdecreasing as only a minimal voltage (e.g., slightly above 0) is beingapplied to the lift pump, and fuel injection is occurring. If thepressure sensor were functioning properly, it would correctly sense theactual delivery pressure reaching the desired trough pressure, and atthat point the controller would increase the voltage applied to the liftpump to a higher voltage. However, because the pressure sensor isreading high, the controller does not increase the voltage applied tothe lift pump to a higher voltage when the actual delivery pressurereaches the desired trough pressure; as shown, at that time, the senseddelivery pressure is still above the desired trough pressure, and thuspulsing of the lift pump to the higher voltage is not triggered. Theactual delivery pressure thus continues to decrease until the senseddelivery pressure reaches the desired trough pressure. In the depictedexample, due to the extent to which the pressure sensor is reading high,the actual delivery pressure decreases to the fuel vapor pressure beforethe sensed delivery pressure has decreased to the desired troughpressure. When the actual delivery pressure reaches the fuel vaporpressure, it cannot decrease further, and thus the remains constant atthe fuel vapor pressure. The sensed delivery pressure also remainsconstant, but at a higher value, as shown. As the higher value isgreater than the desired trough pressure, the controller does not doesnot increase the voltage applied to the lift pump to a higher voltage,and thus the actual delivery pressure remains stuck at the fuel vaporpressure. This can cause the engine to stall. A similar issue may occurif the fuel vapor pressure is higher than the fuel vapor pressure valuestored at the controller. For example, if the actual fuel vapor pressurehas increased above the desired trough pressure (which may occur due toa rapid increase in fuel temperature), the sensed pressure will notdecrease to the desired trough pressure even if the pressure sensor isfunctioning properly. Here again, the controller will not increase thevoltage applied to the LPP to a higher voltage as it is waiting for thedelivery pressure to decrease to the desired trough pressure, which canresult in an engine stall.

In example graph 350 shown in FIG. 3E, the pressure sensor is degradedand reads low (represented by waveform 315) compared with the actualdelivery pressure (represented by waveform 314). In this example, thesecond exemplary feedback control strategy is being performed. At thebeginning of the plot, the delivery pressure is decreasing as only aminimal voltage (e.g., slightly above 0) is being applied to the LPP,and fuel injection is occurring. If the pressure sensor were functioningproperly, it would correctly sense the actual delivery pressure reachingthe desired trough pressure, and at that point the controller wouldincrease the voltage applied to the LPP to a higher voltage. However,because the pressure sensor is reading low, the controller increases thevoltage applied to the LPP to a higher voltage when the sensed deliverypressure reaches the desired trough pressure, which occurs before theactual delivery pressure has decreased to the desired trough pressure.The actual delivery pressure thus does not reach the desired troughpressure, and instead begins increasing in response to the LPP beingpulsed to the higher voltage. In the depicted example, due to the extentto which the pressure sensor is reading low, the actual deliverypressure increases to the pressure relief valve setpoint pressure beforethe sensed delivery pressure has increased to the desired peak pressure.When the actual delivery pressure reaches the pressure relief valvesetpoint pressure, it cannot increase further, and thus remains constantat the pressure relief valve setpoint pressure. The sensed deliverypressure also remains constant, but at a lower value, as shown. As thelower value is lower than the desired peak pressure, the controllercontinues to apply the higher voltage to the LPP, and thus the actualdelivery pressure remains stuck at the pressure relief valve setpointpressure. This will disadvantageously result in increased fuelconsumption and reduced durability of the fuel system, as the deliverypressure is kept higher than required for current engine operatingconditions.

In the examples of sensor degradation described above, the pressuresensor (e.g., sensor 234 or 235 of FIG. 2) may read within theinstrument's operating range, and the error will not be detected bypreviously described methods. In contrast, in accordance with thepresent disclosure, a flattening of the delivery pressure waveform(e.g., the delivery pressure remaining constant for greater than athreshold duration) can indicate pressure sensor degradation even whenthe pressure sensor output is within its normal operating range, asdescribed further herein. What is more, the detection of such flatteningalone can indicate pressure sensor degradation, such that detection ofother parameters (e.g., magnitudes of the sensed delivery pressure) maynot be needed. Accordingly, the control performed for pressure sensordiagnosis may advantageously be simplified.

Referring now to FIG. 4, an example routine 400 is shown for diagnosingan in-range error of a pressure sensor arranged downstream of a fuellift pump in a fuel system. Instructions for carrying out routine 400and the other routines disclosed herein (e.g., routines 500, 510, 530,600, 700, and 1100) may be executed by a controller (such as controller12 of FIG. 1) based on instructions stored in non-transitory memory ofthe controller and in conjunction with signals received from sensors ofthe engine, such as the sensors described above with reference to FIGS.1-2. In carrying out the routines disclosed herein, the controller maysend signals to various engine actuators to adjust engine operation, asdescribed below.

At 402, the routine includes performing closed-loop feedback control ofvoltage pulses applied to the lift pump. The feedback control of thevoltage pulses includes the controller receiving feedback from apressure sensor downstream of the lift pump (e.g., pressure sensor 234or 235 of FIG. 2) and adjusting the voltage applied to the lift pump(e.g., via adjustment of an actuator of the lift pump) based on thefeedback from the pressure sensor. The feedback control may be performedin accordance with the first or second exemplary feedback controlstrategy discussed herein or another control strategy.

At 404, the routine includes determining whether entry conditions fordiagnosing a pressure sensor in-range error are met. The entryconditions may include pressure sensor output existing within apredetermined normal operating range. For example, if the pressuresensor is degraded such that the output is outside of the normaloperating range (e.g., an out-of-range pressure sensor error), in-rangeerror diagnosis is not necessary. When an out-of-range error occurs, acorresponding OBD flag may be set at the controller, and thus,determining whether the entry conditions for the in-range errordiagnosis are met may include the controller verifying the state of thatOBD flag. Further, the entry conditions may include steady state engineoperation and/or engine temperature (e.g., engine coolant temperature)exceeding a threshold. If the entry conditions are not met, for example,due to the presence of an out-of-range sensor error, the routine ends.Otherwise, the routine progresses to 406.

At 406, the routine includes sensing the delivery pressure of the liftpump with a pressure sensor. This may include continually sensing thedelivery pressure of the lift pump throughout operation of the engine.After 406, the routine progresses to 408.

At 408, the routine includes the controller monitoring the senseddelivery pressure for flattening, for example, in accordance with theroutine of FIG. 7, which is discussed below.

If flattening is detected at 410, the routine progresses to 412 andindicates an in-range error of the pressure sensor. In one example,indication of a pressure sensor in-range error may include thecontroller setting an OBD flag. Further, at 412, the routine includesswitching the fuel lift pump from a closed-loop control scheme to anopen-loop control scheme in which the lift pump is energized with acontinuous non-zero voltage and pressure sensor feedback is notconsidered. Switching to open-loop lift pump control allows the fuelsystem to continue operating even when the pressure sensor is degraded,albeit with lower efficiency compared to closed-loop lift pump operationwhen the pressure sensor is not degraded. After 412, the routine ends.

Returning to 410, if flattening is not detected, the routine progressesto 414. At 414, the controller maintains closed-loop control of the liftpump. After 414, routine 400 ends.

Turning now to FIG. 5A, an example routine 500 is shown for performingclosed-loop control of a fuel lift pump.

At 502, routine 500 includes measuring or estimating the engineoperating conditions (e.g., fuel composition, fuel flow rate from theinjectors, and current delivery pressure of the lift pump).

At 504, the routine includes determining the setpoint pressure and fuelvapor pressure. In one example, the setpoint pressure and fuel vaporpressure may be dynamically learned by the controller in the mannerdescribed below with reference to FIG. 6. In another example, thesetpoint pressure may have a predetermined value that is stored innon-transitory memory of the controller, where the predetermined valueis based on characteristics of the pressure relief valve (e.g., pressurerelief valve 211 of FIG. 2) as well as characteristics of the fuelsystem, and the fuel vapor pressure may be calculated as a function ofsensed fuel temperature and fuel composition.

At 506, the routine includes determining the desired peak and troughdelivery pressures of the lift pump. The desired peak delivery pressureis a desired maximum pressure output by the lift pump, whereas thedesired trough delivery pressure is a desired minimum pressure output bythe lift pump. The desired peak delivery pressure may be below thesetpoint pressure by a predetermined margin; similarly, the desiredtrough delivery pressure may be above the fuel vapor pressure by apredetermined margin.

At 508, the routine includes performing closed-loop feedback control ofthe lift pump to achieve the desired peak and trough delivery pressures,for example in accordance with the first exemplary feedback controlstrategy described herein (see FIG. 5B), the second exemplary feedbackcontrol strategy described herein (see FIG. 5C), or the third exemplaryfeedback control strategy described herein (see FIG. 11). After 508, theroutine ends.

FIG. 5B shows an example routine 510 for performing the first exemplaryfeedback control strategy described herein. Routine 510 may be performedin conjunction with routine 500 of FIG. 5A at 508, for example.

At 512, the routine includes determining the magnitude of non-zerovoltage pulses to apply to the lift pump and the duty cycle of thepulses that will produce the desired peak and trough delivery pressuresdetermined in routine 500 at 506. For example, the voltage and/or dutycycle may be determined at the controller via a lookup table stored innon-transitory memory of the controller that indicates the appropriatevoltage and duty cycle given values of parameters such as the fuel vaporpressure, setpoint pressure, desired peak and trough delivery pressuresof the lift pump, fuel injection rate, DI pumping rate, etc.Alternatively, the voltage and/or duty cycle may be determined at thecontroller via functions that receive values of parameters (e.g., fuelvapor pressure, setpoint pressure, desired peak and trough deliverypressures, fuel injection rate, DI pumping rate, etc.) as inputs, andoutput the appropriate voltage and/or duty cycle for the pulses. Theindicated voltage and duty cycle may be selected such that each voltagepulse applied to the lift pump increases delivery pressure to thedesired peak pressure, and such that once the delivery pressuredecreases from the desired peak pressure to the desired trough pressure,the next voltage pulse is applied. In some examples, the same non-zeroeffective voltage is always applied during pulsed operation of the liftpump, whereas the duty cycle of the pulses is varied as engine operatingconditions change.

At 514, the routine includes applying voltage pulses to the lift pump,the pulses having the magnitude and duty cycle determined at 512. Forexample, the controller may send a signal to an actuator of the liftpump which in turn applies voltage pulses having the determinedmagnitude to the lift pump with the determined duty cycle.

At 516, the routine includes monitoring the delivery pressure of thelift pump (e.g., with a pressure sensor such as pressure sensor 234 or235 of FIG. 2). The delivery pressure of the lift pump may be monitoredover a duration, such as a duration that begins when a voltage pulse isapplied and ends when the next voltage pulse is applied. Alternatively,the delivery pressure of the lift pump may be monitored continuouslythroughout engine operation.

After 516, the routine proceeds to 518 to determine whether the sensedpeak and trough delivery pressures are within a predetermined range of(e.g., approximately equal to) the desired peak and trough deliverypressures of the lift pump, respectively. Determining whether the sensedpeak and trough delivery pressures are within the predetermined rangemay include computing, at the controller, a difference between thesensed peak delivery pressure and the desired peak delivery pressure andcomparing the absolute value of the difference to a threshold, andcomputing, at the controller, a difference between the sensed troughdelivery pressure and the desired trough delivery pressure and comparingthe absolute value of the difference to a threshold. If it is determinedat 514 that the sensed peak and trough delivery pressures are within thepredetermined range of the desired peak and trough delivery pressures,the routine progresses to 520, and the controller maintains currentoperation (e.g., continues to perform closed-loop control of the fuellift pump without adjustment of the duty cycle/voltage of the pulses).Following 520, routine 500 returns.

However, if it is determined at 518 that the sensed peak and troughdelivery pressures are not approximately equal to the desired peak andtrough delivery pressures, the routine progresses to 518. At 518, theroutine includes determining whether the sensed peak and trough deliverypressures are greater than the desired peak and trough deliverypressures, respectively (e.g., greater by more than a predeterminedamount).

If the sensed peak and trough delivery pressures are greater than thedesired peak and trough delivery pressures, respectively, the routineprogresses to 524, and the duty cycle of the pulses applied to the liftpump is decreased. For example, the controller may send a signal to anactuator of the lift pump to decrease the duty cycle of the voltagepulses applied to the lift pump. The decrease in the duty cycle may beselected by the controller to be proportional to the difference betweenthe sensed peak and trough delivery pressures and the desired peak andtrough delivery pressures, in some examples. In this way, the controllermay decrease the overall amount of voltage supplied to the fuel liftpump, thereby decreasing the delivery pressure of the lift pump.Following 524, the routine returns.

Returning to 522, if it is instead determined that the sensed peak andtrough delivery pressures are less than the desired peak and troughdelivery pressures, respectively, the routine progresses to 526, and theduty cycle of the pulses applied to the lift pump is increased. Forexample, the controller may send a signal to an actuator of the liftpump to increase the duty cycle of the voltage pulses applied to thelift pump. The increase in the duty cycle may be selected by thecontroller to be proportional to the difference between the sensed peakand trough delivery pressures and the desired peak and trough deliverypressures, in some examples. In this way, the controller may increasethe overall amount of voltage supplied to the fuel lift pump, therebyincreasing the peak and trough delivery pressures of the lift pump.Following 526, the routine returns.

In some examples, routine 500 may be performed in an iterative mannerduring closed-loop control of the lift pump, which allows the controllerto continuously adjust the amount of voltage applied to the fuel liftpump as the desired peak and trough delivery pressures vary.

FIG. 5C shows an example routine 530 for performing the second exemplaryfeedback control strategy described herein. Routine 530 may be performedin conjunction with routine 500 of FIG. 5A at 508, for example.

At 532, the routine includes determining higher and lower voltage levelsto be applied to the lift pump during pulsed operation. The highervoltage level may be a predetermined voltage level which will quicklyraise the delivery pressure to the desired peak pressure (e.g., 8-12 V),whereas the lower voltage level may be a predetermined voltage levelwhich is low enough to keep the lift pump energized (e.g., greater than0 V and less than 0.3 V) and which does not substantially increase fuelpressure. When the higher voltage level is applied to the lift pump, thelift pump may be considered to be in an ON state, whereas when the lowervoltage level is applied to the lift pump, the lift pump may beconsidered to be in an OFF state, despite the fact that a minimal amountof voltage is still being applied.

After 532, the routine proceeds to 534 and the controller applies thedetermined higher voltage to the lift pump.

After 534, the routine proceeds to 536 and the controller determineswhether the sensed delivery pressure is equal to the desired peakdelivery pressure. If not, the routine continues monitoring the senseddelivery pressure until it is equal to the desired peak deliverypressure. As discussed above with reference to FIG. 3E, if the pressuresensor is malfunctioning and reads low, this may result in the senseddelivery pressure never reaching the desired peak delivery pressure. Inthis case, the routine would be stuck at 534 and fuel economy and fuelsystem durability would be negatively affected.

Once the controller determines that the sensed delivery pressure untilit is equal to the desired peak delivery pressure, the routine proceedsto 538 and the controller determines whether the sensed deliverypressure is equal to the desired trough delivery pressure. If not, theroutine continues monitoring the sensed delivery pressure until it isequal to the desired trough delivery pressure. As discussed above withreference to FIG. 3D, if the pressure sensor is malfunctioning and readshigh, this may result in the sensed delivery pressure never reaching thedesired trough delivery pressure. In this case, the routine would bestuck at 538 and the engine could potentially stall due to lack ofadequate fuel pressure.

It will be appreciated that performance of routine 530 may beinterrupted and/or suspended by the controller in order to switch to adifferent fuel system control strategy or turn off the engine.

FIG. 6 shows an example routine 600 for determining the setpointpressure (e.g., the physical maximum pressure in the fuel system forcurrent engine operating conditions) and the fuel vapor pressure (e.g.,the physical minimum pressure in the fuel system for current engineoperating conditions). In accordance with routine 600, the controllermay initiate a determination of the setpoint pressure during engineoperating conditions where the desired peak and trough deliverypressures of the lift pump are relatively high. Further, the controllermay initiate a determination of the fuel vapor pressure when the desiredpeak and trough delivery pressures of the lift pump are relatively low.In this way, dynamic learning of the setpoint pressure and fuel vaporpressure may be performed intermittently during engine operation in amanner that takes advantage of fluctuations in the desired peak andtrough delivery pressures of the lift pump, so as to minimize activeadjustments to engine operation associated with performing the dynamiclearning.

At 602, the routine begins by measuring and/or estimating the engineoperating conditions, for example, in the manner described above forroutine 500 at 502.

At 604, the routine includes determining if the engine is operating insteady state and warmed up. For example, it may be determined that theengine is operating in steady state if the engine speed remainssubstantially constant for at least a threshold duration. Further, itmay be determined that the engine is warmed up if engine temperature isdetermined to be greater than a threshold temperature (e.g., based onoutput from an engine coolant temperature sensor). Routine 600 returnsif the engine is not warmed up and in steady state operation. Otherwise,if the engine is warmed up and operating in steady state, the routineprogresses to 606.

At 606, the routine includes determining if the entry conditions forlearning the setpoint pressure are met. In one example, the entryconditions for learning the setpoint pressure include the peak deliverypressure being greater than a threshold and/or the trough deliverypressure being greater than a threshold. In another example, the entryconditions for learning the setpoint pressure include the engine loadbeing greater than a threshold. If at 606 it is determined that theentry conditions for learning the setpoint pressure are not met, theroutine proceeds to 608 to determine if the entry conditions forlearning the fuel vapor pressure are met, which will be explained infurther detail below. Otherwise, if the entry conditions for learningthe setpoint pressure are met at 606, the routine proceeds to 610.

At 610, the routine includes increasing the duty cycle of voltage pulsesapplied to the fuel lift pump until the sensed delivery pressure of thepump flattens. Flattening may be determined in accordance with routine700 of FIG. 7, which is discussed below. Flattening of the senseddelivery pressure represents hitting a physical limit of the fuelsystem. In this example, the physical limit is the setpoint pressure.The pressure in the fuel system cannot exceed this pressure; when thepressure in the fuel system reaches the setpoint pressure, the pressurerelief valve opens and fuel flows back to the fuel tank. The pressurerelief valve remains open until the pressure in the fuel systemdecreases to the setpoint pressure, at which point the pressure reliefvalve closes.

At 612, the routine includes setting the setpoint pressure to thepressure at which the flattening occurred. In this way, the controllerlearns the maximum possible delivery pressure. Because the fuel pressuresensor may clog or otherwise degrade, for example, this value may changeover time. Therefore, it is advantageous for the controller toperiodically relearn this value. As one example, knowing the maximumpressure of the system may help the controller distinguish in-rangepressure sensor error, as described in detail below with reference toFIG. 7. Furthermore, knowing the setpoint pressure with high accuracymay enable the controller to set the desired peak delivery pressure tobe below the setpoint pressure by a small margin (e.g., 20 kPa). In onenon-limiting example, if the setpoint pressure is determined to be 650kPa, the desired peak delivery pressure may be set to 630 kPa. As such,the duty cycle of the voltage pulses applied to the fuel lift pump toachieve the desired peak delivery pressure may be reduced, therebyimproving fuel economy.

After 612, the routine proceeds to 614. At 614, the routine includesreturning to normal closed-loop control of the lift pump (e.g., byexecuting routine 500 of FIG. 5A). For example, this may include thecontroller determining a duty cycle of lift pump activation that willadjust the peak delivery pressure to the desired peak delivery pressureand controlling an actuator of the lift pump to adjust the duty cycle ofthe voltage pulses applied to the lift pump to the determined dutycycle. The adjustment may include decreasing the duty cycle of the liftpump voltage pulses so that the delivery pressure remains below thesetpoint pressure. After 614, the routine progresses to 608.

At 608, the routine includes determining if the entry conditions forlearning the fuel vapor pressure are met. In one example, the entryconditions for learning the fuel vapor pressure include the peakdelivery pressure being less than a threshold and/or the trough deliverypressure being less than a threshold. In another example, the entryconditions for learning the fuel vapor pressure include the engine loadbeing less than a threshold. If at 608 it is determined that the entryconditions for learning the fuel vapor pressure are not met, routine 600ends. Otherwise, if the entry conditions for learning the fuel vaporpressure are met, the routine proceeds to 616.

At 616, the routine includes decreasing the duty cycle of the voltagepulses applied to the fuel lift pump until the delivery pressure of thepump flattens (e.g., as measured by pressure sensor 234 or 235 of FIG.2). Flattening may be determined in accordance with routine 700 of FIG.7, which is discussed below. Flattening of the sensed delivery pressurerepresents hitting a physical limit of the fuel system. In this example,the physical limit is the fuel vapor pressure.

At 618, the routine includes setting the fuel vapor pressure to thepressure at which flattening occurs, as determined at 616. In this way,the controller learns the minimum delivery pressure possible for thefuel system. Fuel temperature may fluctuate during vehicle operation,thereby changing the fuel vapor pressure. Determining the fuel vaporpressure in accordance with routine 600 may be more accurate thancomputing the fuel vapor pressure based on sensed or inferred fuelcomposition and temperature. Knowing the fuel vapor pressure at a giventime with high accuracy may enable the fuel system to operate at a smallpressure above the fuel vapor pressure without the risk of losing thedesired pressure margin between the vapor pressure and the injectionpressure due to temperature fluctuation. For example, this method couldbe used in place of the Hot Injector Compensation method, wherein lessfuel is metered than intended due to operating at a larger pressure(e.g., 50 or 100 kPa) above the fuel vapor pressure. Further, knowingthe minimum pressure of the system may help the controller distinguishin-range pressure sensor error, as described further below withreference to FIG. 7.

At 620, the routine includes returning to normal closed-loop control ofthe lift pump (e.g., by executing routine 500 of FIG. 5A). For example,this may include the controller determining a duty cycle of lift pumpactivation that will adjust the peak delivery pressure to the desiredpeak delivery pressure and controlling an actuator of the lift pump toadjust the duty cycle of the voltages pulses applied to the lift pump tothe determined duty cycle. The adjustment may include increasing theduty cycle of the lift pump voltage pulses so that the delivery pressureremains above the fuel vapor pressure. After 620, the routine returns.

FIG. 7 shows an example routine 700 for diagnosing an in-range fuelpressure sensor error. An in-range error may occur when the output ofthe pressure sensor appears is within an expected range (e.g., thesensor output voltage is non-zero and an industry standard rangeout-of-range check does not indicate that the output is out-of-range).When an in-range error is occurring, the output of the pressure sensorcorresponds to a pressure that is higher or lower than the actualdelivery pressure but still within a normal pressure range of the fuelsystem.

At 702, the routine includes determining a threshold duration for thesensed pressure to remain constant during proper sensor operation. Forexample, as discussed above with reference to FIGS. 3A-3E, during pulsedoperation of the lift pump, the delivery pressure fluctuates in asawtooth pattern which includes sharp peaks and troughs. The thresholdduration may be a longest duration at which the pressure is expected toremain at the peak or trough pressure for current operating conditions.The threshold duration may be determined empirically, e.g. duringmanufacturing of the vehicle, and stored in non-transitory memory of thecontroller, e.g. in a lookup table which stores threshold durationscorresponding to different operating conditions such as different pulsewidths of the voltage pulses applied to the lift pump. As discussedbelow with reference to FIG. 9, the threshold duration at a given timemay be substantially less than the pulse width of the voltage pulsesapplied to the lift pump at that time.

At 704, the routine includes monitoring the sensed delivery pressure(e.g., as measured by sensor 234 or 235 of FIG. 2). In some examples,the monitoring may be stopped as soon as the sensed delivery pressureremains constant for greater than the threshold duration, even if thathappens before the end of the application of the first voltage pulseduring the monitoring. In other examples, the monitoring may performedduring application of a predetermined number of voltage pulses to thelift pump regardless of whether the sensed delivery pressure remainsconstant for greater than the threshold duration before thepredetermined number of voltage pulses have all been applied. Thepredetermined number may be one, two, three or any other number ofvoltage pulses.

At 706, the routine includes determining if the sensed delivery pressurehas remained constant for more than a threshold duration, e.g. thethreshold duration determined at 702. In some examples, the senseddelivery pressure remaining constant for more than the thresholdduration generates an interrupt. In response to a determination that thesensed delivery pressure has remained constant for more than thethreshold duration, the routine proceeds to 708 and the controllerindicates an in-range error (e.g., by setting an OBD flag). Followingstep 708, the routine returns.

Returning to 706, if the sensed pressure does not remain constant formore than a threshold duration during the monitoring, the routineprogresses to 710, and the controller indicates that there is noin-range error of the pressure sensor (e.g., by not setting an OBDflag). Following 710, the routine returns.

Turning now to FIG. 8, it shows an example map 800 illustrating signalsof interest during dynamic learning of the setpoint pressure and fuelvapor pressure of a fuel system, e.g. in accordance with routine 600 ofFIG. 6. Map 800 depicts the setpoint pressure at plot 802, the fuelvapor pressure at plot 804, the desired (e.g., commanded) deliverypressure at plot 806, the voltage applied to the lift pump at plot 808,the sensed delivery pressure of the lift pump at plot 810, the engineload at plot 812, and the engine temperature at plot 814. For all of theabove plots, the X-axis represents time, with time increasing along theX-axis from left to right. The Y-axis of each individual plotcorresponds to the labeled parameter, with the value increasing frombottom to top. Additionally, line 816 represents a first higherthreshold value for engine load, line 818 represents a second lowerthreshold value for engine load, and line 820 represents a thresholdvalue for engine temperature.

The expected physical behavior of the fuel system is that the pressurerelief valve setpoint pressure 802 is constant over life. In contrast,the fuel vapor pressure 804 is dependent on fuel composition andstrongly linked with fuel temperature. Thus, it will changesignificantly as the vehicle warms up with operation. However, betweenthe fuel composition specification and design actions, the maximum fuelvapor pressure is expected to be limited to a worse case value. Innormal operation, the desired peak pressure is set to be below pressurerelief valve setpoint pressure 802 and the desired trough pressure isset to be above fuel vapor pressure 804. However, to discover the valuesof each, the controller may purposely violate that normal objective.

Between t0 and t1, the fuel lift pump may be operated under aclosed-loop control scheme, e.g. in accordance with routine 500 of FIG.5A. The controller, such as controller 12 of FIG. 1, sends a signal toan actuator of the lift pump which causes the actuator to apply non-zerovoltage pulses to the lift pump at a duty cycle which will produce adesired delivery pressure characteristic 806. As shown, desired deliverypressure characteristic 806 may vary with engine load. The energizingvoltage pulses applied to the lift pump to obtain the desired deliverypressure characteristic 806 are shown at plot 808. The delivery pressureof the fuel lift pump, as measured by a sensor (e.g., pressure sensor234 or 235 of FIG. 2) and illustrated at plot 810, increases in responseapplication of voltage to the lift pump. Between energizing pulses, whenzero voltage is applied to the lift pump, the delivery pressure of thelift pump decreases due to fuel consumption by the engine.

It may be favorable to dynamically learn the fuel vapor pressure andsetpoint pressure in order to maximize fuel economy, as described indetail above with reference to FIG. 6. In order to proceed with dynamiclearning of either the fuel vapor pressure or the setpoint pressure,however, the engine must be in steady state operation and warmed up, andthe corresponding entry conditions must be met. In example shown in map800, between t0 and t1, the engine is operating in steady state, andthus engine load 812 remains substantially constant. Further, the enginetemperature 814 is greater than the threshold value represented bydashed line 820, indicating that the engine is warmed up. Furthermore,engine load is above the first higher threshold value 816. Therefore, att1, the entry conditions for learning the setpoint pressure are met. Inother examples, however, additional entry conditions may need to be metbefore learning the setpoint pressure.

Between t1 and t2 of map 800, the controller learns the setpointpressure 802, which is the maximum delivery pressure possible in thefuel system due to the presence of the pressure relief valve. As shown,in order to determine the maximum delivery pressure, the controllerincreases the duty cycle of the voltage pulses 808 applied to the fuellift pump at t1. This increase is not responsive to change in engineoperating conditions (e.g., an increase in engine load), or an increasein desired (e.g., requested or commanded) delivery pressure; rather, theincrease is performed for the sole purpose of determining the maximumdelivery pressure of the fuel system, which corresponds to the setpointpressure of the pressure relief valve. For example, despite engine load812 remaining substantially constant between time t0 and time t1, thecontroller nonetheless increases the duty cycle of the voltage pulsesapplied to the fuel lift pump in order to perform the dynamic learningof the setpoint pressure.

When the sensed delivery pressure 810 reaches the setpoint pressure 802,the waveform of the sensed delivery pressure develops a flattened peakcharacteristic. In the depicted example, the delivery pressure reachesthe setpoint pressure during application of the first voltage pulsehaving an increased pulse width. In other examples, however, the rampingup of the duty cycle may be performed incrementally, such that thesensed delivery pressure does not reach the setpoint pressure untilmultiple voltage pulses have been applied, advantageously reducing theabruptness of the increase in pressure delivered to the fuel injectors.Further, incremental ramping up of the duty cycle provides for flattenedpeak detection while minimizing the delivery pressure increase, suchthat the delivery pressure remains closer to the optimal deliverypressure for current engine operating conditions.

In map 800, the setpoint pressure is 650 kPa, and the sensed deliverypressure waveform flattens (remains constant) at 650 kPa for anon-trivial duration. This particular setpoint pressure is only anexample; the setpoint pressure will vary depending on thecharacteristics of the pressure relief valve and fuel system.

In the depicted example, the controller continues to monitor the senseddelivery pressure after flattening is detected; specifically, anothervoltage pulse is applied after once instance of flattening has beensensed, such that the delivery pressure signal flattens twice at thepeak. Applying one or more additional voltage pulses with an increasedpulse width relative to the nominal pulse width even after detecting afirst instance of flattening may be advantageous in that it may reduce afalse positive detection of flattening (e.g., when an anomaly occursresulting in temporary flattening of the sensed delivery pressure signalwhich is not indicative of the actual setpoint pressure). However, inother examples, the controller may end the setpoint pressure learningprocedure as soon as flattening is detected, and update the storedsetpoint pressure to be the pressure at which the flattening occurred.This may limit the duration of time during which the voltage applied tothe lift pump is increased by the controller to perform the learning,and therefore improve fuel economy.

Upon detection of the flattening of the sensed delivery pressurewaveform, the controller compares the pressure at which the senseddelivery pressure has flattened with a previously determined setpointpressure stored in non-transitory memory. As the setpoint pressure issubject to change over time (e.g., as the pressure relief valve clogs,or as other parameters of the fuel system change), it may be desirableto re-learn the setpoint pressure periodically; towards this end,routine 600 may be performed intermittently, or optionally continuously,during pulsed operation of the lift pump. In other examples, routine 600may be performed only when pulsed operation of the lift pump isinitiated.

At t2, the controller ends the setpoint pressure learning process andswitches operation of the lift pump back to the closed-loop controlscheme described in routine 500 of FIG. 5A. For example, as shown, thecontroller may decrease the duty cycle of the voltage pulses 808 appliedto the lift pump to a value which reflects current engine operatingparameters (e.g., the same value applied from time t0 to t1).

Between t2 and t3, engine load 812 decreases to a level which is lowerthan second threshold 818. The decrease in engine load may occur due toa change in engine operation (e.g., a transition to idling operation, ordownhill travel of the vehicle). Further, the engine temperature 814 isgreater than the threshold value represented by dashed line 820,indicating that the engine is warmed up. Therefore, at t3, the entryconditions for learning the fuel vapor pressure are met. In otherexamples, however, additional entry conditions may need to be met beforelearning the fuel vapor pressure. For example, as fuel vapor pressure isa function of fuel temperature in the fuel system, the entry conditionsmay include fuel temperature remaining substantially constant for atleast a threshold duration.

Upon determining that the entry conditions have been met at t3, thecontroller alters operation of the lift pump in order to learn the fuelvapor pressure by decreasing the duty cycle of the voltage pulses 808applied to the fuel lift pump. This decrease is not responsive to achange in engine operating conditions (e.g., a decrease in engine load),or a decrease in desired (e.g., requested or commanded) deliverypressure; rather, the decrease is performed for the sole purpose ofdetermining the maximum delivery pressure of the fuel system, whichcorresponds to the setpoint pressure of the pressure relief valve. Forexample, despite engine load 812 remaining substantially constant fromthe time period immediately prior to t3 until t3, the controllernonetheless decreases the duty cycle of the voltage pulses applied tothe lift pump at t3 in order to perform the dynamic learning of the fuelvapor pressure.

When the delivery pressure 810 reaches the fuel vapor pressure 804, thewaveform of the sensed delivery pressure develops a flattened troughcharacteristic. In the depicted example, the delivery pressure reachesthe fuel vapor pressure after application of the first voltage pulsehaving a decreased pulse width, and prior to application of a secondvoltage pulse having a decreased pulse width. In other examples,however, the decreasing of the duty cycle may be performedincrementally, such that the sensed delivery pressure does not reach thefuel vapor pressure until multiple voltage pulses have been applied,advantageously reducing the abruptness of the decrease in pressuredelivered to the fuel injectors. Further, incremental decreasing of theduty cycle provides for flattened trough detection while minimizing thedelivery pressure decrease, such that the delivery pressure remainscloser to the optimal delivery pressure for current engine operatingconditions.

In map 800, the fuel vapor pressure is 300 kPa, and the sensed deliverypressure waveform flattens (remains constant) at 300 kPa for anon-trivial duration. This particular fuel vapor pressure is only anexample; the fuel vapor pressure will vary depending on the operatingconditions of the fuel system (e.g., fuel temperature). Therefore, itmay be beneficial to re-learn the fuel vapor pressure periodically.

In the depicted example, the controller continues to monitor the senseddelivery pressure after flattening is detected; specifically, anothervoltage pulse is applied after once instance of flattening has beensensed, such that the delivery pressure signal flattens twice at thetrough. As discussed above with reference to learning the setpointpressure, applying one or more additional voltage pulses with adecreased pulse width relative to the nominal pulse width even afterdetecting a first instance of flattening may be advantageous in that itmay reduce a false positive detection of flattening (e.g., when ananomaly occurs resulting in temporary flattening of the sensed deliverypressure signal which is not indicative of the actual fuel vaporpressure). However, in other examples, the controller may end the fuelvapor pressure learning procedure as soon as flattening is detected, andupdate the stored fuel vapor pressure to be the pressure at which theflattening occurred. This may limit the duration of time during whichthe delivery pressure is modified from the requested delivery pressurein order to perform learning, and therefore improve engine operation.

Upon detection of the flattening of the sensed delivery pressurewaveform, the controller stores the pressure at which the senseddelivery pressure has flattened in non-transitory memory as the fuelvapor pressure. Routine 600 may be performed intermittently, oroptionally continuously, during pulsed operation of the lift pump, inorder to improve accuracy of the closed-loop control. In other examples,routine 600 may be performed only when pulsed operation of the lift pumpis initiated.

After learning the fuel vapor pressure, the controller switchesoperation of the lift pump back to the closed-loop control schemedescribed in routine 500 of FIG. 5A. For example, the controller mayadjust the duty cycle of the voltage pulses 808 applied to the lift pumpto a value which reflects current engine operating parameters (e.g., thecurrent desired delivery pressure characteristic of the lift pumprepresented by plot 806).

While map 800 illustrates dynamic learning of the setpoint pressurefollowed shortly thereafter by dynamic learning of fuel vapor pressure,this sequence of events is only exemplary. Dynamic learning of thesetpoint pressure may be performed any time corresponding entryconditions (including engine load above the first higher threshold) aremet, and similarly, dynamic learning of fuel vapor pressure may beperformed any time the corresponding entry conditions (including engineload below the second lower threshold) are met.

In examples where the vehicle including the fuel system is a hybridvehicle, engine load may be increased or decreased when it is desired tolearn the setpoint pressure or fuel vapor pressure even when engine loadis not in the appropriate range (e.g., above the first higher thresholdor below the second lower threshold), by adding or removing some amountof load from the engine via the electric machine and the battery. Forexample, rather than waiting until engine load exceeds the first higherthreshold to learn the setpoint pressure, engine load may be increasedto above the first higher threshold, and the excess engine output may beconverted into electrical energy via the electric machine (operating ina generating mode) and stored in the energy storage device. Conversely,rather than waiting until engine load falls below the second lowerthreshold to learn the fuel vapor pressure, engine load may be decreasedto below the second lower threshold, and the battery and electricmachine (operating in a motor mode) may provide supplemental torque tovehicle wheels, such that the requested torque is still provided tovehicle wheels despite the decrease in engine load.

Further, in examples where the vehicle including the fuel system is ahybrid vehicle, and the robust feedback control strategy is performed, avolume of fuel ingested by the engine may be monitored while the liftpump is OFF. If the volume of fuel ingested by the engine while the liftpump is OFF reaches a predetermined volume before an output signal ofthe pressure sensor has decreased to a desired trough pressure, the liftpump may be turned ON, the value of the output signal of the pressuresensor may be stored as a first stored value, dynamic learning of a fuelvapor pressure of the fuel system may be requested. As discussed above,if the volume of fuel ingested by the engine while the lift pump is OFFexceeds an expected amount for current operating conditions, yet thedesired trough pressure has not yet been reached, this indicates thateither the sensor is inaccurate, or the fuel vapor pressure has changed(e.g., has risen to above the desired trough pressure). In order todiscern which of these issues is present, the controller may performdynamic learning of the fuel vapor pressure by reducing fuel railpressure until it will not reduce further. In order to do so withoutcomprising desired engine operation during conditions where requestedengine output torque is above a threshold, the motor/generator may beused to supplement engine output torque. Thus, if a requested vehiclewheel torque is above a first threshold, the controller may send signalsto actuators to mechanically couple a crankshaft of the engine to themotor/generator and decrease engine load until the output signal of thepressure sensor remains constant for at least a first threshold durationwhile converting electrical energy to torque with the motor/generatorand providing the torque the vehicle wheels. The pressure at which theoutput signal remains constant may then be stored as an updated fuelvapor pressure, and if the updated fuel vapor pressure is less than thefirst stored value, the controller may indicate that the pressure sensoris reading high. In this case, calibration of sensor output maysubsequently be performed, which may take into account the differencebetween the updated fuel vapor pressure and the first stored value.Otherwise, the controller may indicate that the pressure sensor isreading correctly and not degraded, and perform subsequent feedbackcontrol of the lift pump based on the updated fuel vapor pressure.

Similarly, during pulsed operation of the lift pump, an ON time of thelift pump may be monitored; if the ON time of the lift pump reaches acalibrated maximum ON time before the output signal of the pressuresensor has increased to a desired peak pressure, the lift pump may beturned OFF, the value of the output signal of the pressure sensor may bestored as a second stored value, and dynamic learning of a setpointpressure of the pressure relief valve may be requested. As discussedabove, if lift pump remains ON for a calibrated maximum ON time, yet thedesired peak pressure has not yet been reached, this indicates thateither the sensor is inaccurate, or the pressure relief valve setpointhas changed (e.g., has decreased from the stored value). In order todiscern which of these issues is present, the controller may performdynamic learning of the setpoint pressure by increasing fuel railpressure until it will not reduce further. In order to do so withoutcomprising desired engine operation during conditions where requestedengine output torque is below a threshold, the motor/generator may beused to absorb excess engine output torque. Thus, if a requested vehiclewheel torque is below a second threshold, the controller may sendsignals to actuators to mechanically couple the crankshaft to themotor/generator, increase engine load until the output signal of thepressure sensor remains constant for at least a second thresholdduration while converting a portion of engine output torque toelectrical energy with the motor/generator and storing the electricalenergy at the battery, and store the pressure at which the output signalremains constant as an updated setpoint pressure. If the updatedsetpoint pressure is greater than the second stored value, thecontroller may indicate that the pressure sensor is reading low. In thiscase, calibration of sensor output may subsequently be performed, whichmay take into account the difference between the updated setpointpressure and the second stored value. Otherwise, the controller mayindicate that the pressure sensor is reading correctly and not degraded,and perform subsequent feedback control of the lift pump based on theupdated setpoint pressure.

FIG. 9 shows an example map 900 illustrating signals of interest fordiagnosing an in-range error of a pressure sensor sensing deliverypressure of a lift pump, e.g. in accordance with routine 700 of FIG. 7.Map 900 shows the commanded delivery pressure of the fuel lift pump atplot 902, the voltage applied to the lift pump at plot 904, the senseddelivery pressure at plot 906, an indication of whether entry conditionshave been met at plot 912, and an indication of in-range pressure sensorerror at plot 916. Additionally, the setpoint pressure is symbolicallyrepresented as dashed line 908, and the fuel vapor pressure issymbolically represented as dashed line 910. For all of the above, theX-axis represents time, with time increasing along the X-axis from leftto right. The Y-axis of plots 902, 904, and 906 corresponds to thelabeled parameter, with the value increasing from bottom to top.

From t0 to t1, the controller is performing closed-loop control ofvoltage pulses applied to the fuel lift pump (e.g., in accordance withthe first or second exemplary feedback control strategy describedherein). As shown at plot 904, the voltage pulses applied have a pulsewidth 905. The pulsed operation produces a sensed delivery pressurewaveform with a sawtooth shape, as shown at plot 906. Prior to t1, entryconditions for diagnosing an in-range error of the pressure sensor arenot met. For example, engine temperature is below a threshold, theengine is not operating in steady state, and/or other entry conditionsare not met. Further, during this time period an in-range error of thepressure sensor is not indicated (e.g., an OBD flag representing anin-range error of the pressure sensor is not set).

At time t1, as shown at plot 912, the controller indicates that theentry conditions for diagnosing an in-range error of the pressure sensorhave been met (e.g., in response to measured and/or inferred signalsrepresenting values of engine load, engine temperature, etc.). Inresponse to this indication, the controller initiates a routine fordiagnosing an in-range error of the pressure sensor, such as routine 700of FIG. 7. This may include first determining a threshold duration forthe sensed delivery pressure to remain constant during proper sensoroperation. In map 900, an exemplary threshold duration is shown at 918.The threshold duration may optionally be determined at the controller asa function of the pulse width, e.g., as a fraction of the pulse width.For example, the controller may make a logical determination of anappropriate threshold duration for current engine and fuel systemoperating conditions, based on logic rules that are a function of thepulse width. In the depicted example, threshold duration 918 is smallerthan pulse width 905. It will be appreciated that threshold duration maybe substantially smaller than the pulse width (e.g., less than 1/100 ofthe pulse width), without departing from the scope of this disclosure.

From t1 to t2, the controller performs the diagnostic routine bymonitoring the sensed delivery pressure to determine whether it remainsconstant (e.g., flattens) for more than the threshold duration. As shownat plot 906, the sensor is operating in-range with the expected sawtoothoutput signal until shortly before t2. However, an in-range error of thesensor begins to occur shortly before t2; at t2, the sensed deliverypressure has remained constant for the threshold duration. In thisexample, the flattening occurs at the trough of the waveform, which isindicative of the pressure sensor reading high. However, in performingthe diagnosis, the controller may ignore the magnitude of the pressureat which the flattening occurs, and thus not distinguish between troughand peak flattening (e.g., the diagnosis is performed independent of themagnitude of pressure at which the sensed signal remains constant). Suchoperation may advantageously simplify the control strategy.

Upon detecting at t2 that the sensed delivery pressure has remainedconstant for the threshold duration, the controller indicates anin-range error of the pressure sensor, as shown at plot 916. Further, att2, the controller switches from closed-loop control of the lift pump,in which voltage pulses are applied to the lift pump, to open-loopcontrol of the lift pump, in which a continuous non-zero voltage isapplied to the lift pump. For example, as shown at plot 904, acontinuous non-zero voltage is applied to the lift pump starting at t2.In response to the application of the continuous non-zero voltage, thesensed delivery pressure ramps up to a pressure which is higher than anaverage pressure of the sawtooth waveform, and then remainssubstantially constant at that pressure (assuming a constant fuelinjection rate). The delivery pressure may vary in response to varianceof the fuel injection rate that occurs during open-loop operation of thefuel pump, however. By switching to open-loop control of the lift pumpwhen an in-range error of the pressure sensor is identified, thecontroller no longer relies on inaccurate feedback from pressure sensor.This in turn improves robustness of the control of the lift pump, andreduces the possibility of an inadequate amount of fuel being suppliedto the engine cylinders.

In the example shown in map 900, an in-range error of the pressuresensor is indicated as soon as the sensed delivery pressure has remainedconstant for the threshold duration. In other examples, such as theexample shown in map 1000 of FIG. 10, the controller may continue tomonitor the sensed delivery pressure during the application of multiplevoltage pulses, to ensure that the sensed flattening is not a fluke.

Turning now to FIG. 10, it shows another example map 1000 illustratingsignals of interest for diagnosing an in-range error of a pressuresensor sensing delivery pressure of a lift pump, e.g. in accordance withroutine 700 of FIG. 7. Map 1000 shows the commanded delivery pressure ofthe fuel lift pump at plot 1002, the voltage applied to the lift pump atplot 1004, the sensed delivery pressure at plot 1006, an indication ofwhether entry conditions have been met at plot 1012, and an indicationof in-range pressure sensor error at plot 1016. Additionally, thesetpoint pressure is symbolically represented as dashed line 1008, andthe fuel vapor pressure is symbolically represented as dashed line 1010.For all of the above, the X-axis represents time, with time increasingalong the X-axis from left to right. The Y-axis of plots 1002, 1004, and1006 corresponds to the labeled parameter, with the value increasingfrom bottom to top.

From t0 to t1, the controller is performing closed-loop control ofvoltage pulses applied to the fuel lift pump, as described above withregard to map 900. However, whereas the closed-loop control shown in map900 is performed in accordance with either the first or second exemplaryfeedback control strategy, for example, the closed-loop control shown inmap 1000 is not consistent with the second exemplary feedback controlstrategy (seeing as the control does not get “stuck” when the desiredpeak pressure is not reached due to flattening). In other examples,however, an in-range error may be detected when flattening occurs at thepeak during closed-loop control of the lift pump in accordance with thesecond exemplary feedback control strategy.

Prior to t1, entry conditions for diagnosing an in-range error of thepressure sensor are not met, and an in-range error of the pressuresensor is not indicated. However, as shown at plot 1006, an in-rangeerror of the pressure sensor is occurring, as evidenced by theflattening of the peaks of the sensed pressure signal.

At time t1, as shown at plot 912, the controller indicates that theentry conditions for diagnosing an in-range error of the pressure sensorhave been met, and initiates a routine for diagnosing an in-range errorof the pressure sensor, such as routine 700 of FIG. 7. As discussedabove with reference to map 900, this may include first determining athreshold duration for the sensed delivery pressure to remain constantduring proper sensor operation; an exemplary threshold duration is shownat 1018. In the depicted example, threshold duration 1018 is smallerthan pulse width 1005 of the voltage pulses applied to the lift pump.

From t1 to t2, the controller performs the diagnostic routine bymonitoring the sensed delivery pressure to determine whether it remainsconstant (e.g., flattens) for more than the threshold duration. As notedabove, at the time the diagnostic routine is initiated, an in-rangeerror of the sensor is already occurring; upon application of the firstvoltage pulse applied during the diagnostic routine, the sensed deliverypressure rises and then flattens, remaining constant for greater thanthreshold duration 1018. Whereas in the example diagnostic routine shownin map 900 the controller indicates an in-range error as soon asflattening for the threshold duration is detected, map 1000 shows anexample diagnostic routine in the controller waits until multipleseparate instances of flattening have been detected before indicating anin-range error. Specifically, in the depicted example, the controllerdoes not indicate an in-range error until the sensed delivery pressureremains constant for the threshold duration for a third time, whichoccurs at t2. This example is non-limiting; in other examples, thecontroller may wait to indicate an in-range error until flattening hasoccurred one, two, three, fourth, five, or more times. Alternatively,another routine for detecting flattening of the sensed pressure signalmay be performed by the controller without departing from the scope ofthis disclosure.

Upon indicating the in-range error, as in map 900, the controllerswitches from closed-loop control of the lift pump to open-loop controlof the lift pump. For example, as shown at plot 1004, a continuousnon-zero voltage is applied to the lift pump after t2. As shown, thecontinuous voltage does not begin to be applied until the senseddelivery pressure has decreased a certain amount from the flattened peakpressure; such operation may be appropriate as the desired deliverypressure may be less than the peak pressure during pulsed operation ofthe lift pump. However, in other examples, the continuous voltage may beapplied as soon as the in-range error is indicated, or the voltage maybe ramped up to the continuous voltage, or another strategy fortransitioning from pulsed operation to continuous operation of the liftpump may be used. In any case, in response to the application of thecontinuous non-zero voltage, the sensed delivery pressure ramps up to apressure which is higher than an average pressure of the sawtoothwaveform, and then remains substantially constant at that pressure(assuming a constant fuel injection rate). The delivery pressure mayvary in response to variance of the fuel injection rate that occursduring open-loop operation of the fuel pump, however.

Routine 700 and maps 900 and 1000 pertain to diagnosis of an in-rangepressure sensor error and corresponding adjustment of lift pump controlfrom closed-loop control to open-loop control. Alternatively, ratherthan transitioning to open-loop control of the lift pump when anin-range pressure sensor error is occurring, a third exemplary feedbackcontrol strategy may be enacted, which is referred to herein as a robustcontrol.

FIG. 11 shows an example routine 1100 for performing robust control of afuel lift pump in accordance with the third exemplary feedback controlstrategy. This robust control strategy may advantageously allow forclosed-loop feedback control of the lift pump to continue even in theevent of pressure sensor degradation or an increase in fuel vaporpressure which has not been recognized by the controller, whileminimizing stalling and excessive fuel consumption. Routine 1100 may beperformed in conjunction with routine 500 of FIG. 5A at 508, forexample.

At 1102, the routine includes turning the lift pump ON. For example, asdiscussed above with respect to routine 530, this may include thecontroller adjusting an actuator of the lift pump to apply apredetermined higher voltage level to the lift pump which will quicklyraise the delivery pressure to the desired peak pressure (e.g., 8-12 V)determined in routine 500.

After 1102, the routine proceeds to 1104 and the controller determineswhether the sensed delivery pressure is less than the desired peakdelivery pressure. For example, the controller may receive a signal froma pressure sensor which is indicative of the delivery pressure, andcompare this sensed delivery pressure with the stored value of thepreviously determined desired peak delivery pressure. If the answer at1104 is YES, indicating that either the delivery pressure has notreached the desired peak pressure or the sensor output is inaccurate,the routine proceeds to 1106.

At 1106, the controller determines whether the duration of time that thelift pump has been ON is less than a calibrated maximum value. Thecalibrated maximum value may be a predetermined value stored in memory,or alternatively may be determined at the controller as a function ofvarious engine operating parameters (e.g., fuel consumption rate, enginespeed, level of voltage applied to the lift pump, etc.) during executionof routine 1100. The calibrated maximum value represents the maximumduration that the lift pump should remain in the ON state duringconditions where pressure sensor degradation or some other errorprevents the sensed delivery pressure from reaching the desired peakpressure. If the answer at 1106 is YES, the routine returns to 1104.Otherwise, if the answer at 1106 is NO, indicating that the lift pumphas been ON for at least the calibrated maximum duration, the routinereturns to 1102, or optionally proceeds to 1108.

At 1108, the controller calibrates the output of the pressure sensor, soas to produce a more accurate indication of the actual deliverypressure. When the lift pump remains ON for at least the calibratedmaximum duration, this may occur due to flattening of the signal fromthe sensor which reflects the actual delivery pressure being equal tothe setpoint pressure of the pressure relief valve. Such flattening maybe determined in accordance with the method of FIG. 7, for example. Inone exemplary calibration strategy, upon determination that the liftpump has remained ON for the calibrated maximum duration, the controllerproceeds to determine whether the sensed delivery has remained constantfor more than a threshold duration (e.g., flattened). If so, goingforward, the controller determines a pressure offset as the differencebetween the pressure relief valve setpoint pressure and the pressure atwhich the sensed delivery pressure flattened, and calibrates the outputof the pressure sensor by adding the offset to the sensed deliverypressure. Thus, the calibrated delivery pressure generated at thecontroller at a given time may be equal to the sum of the offset and thecurrently sensed delivery pressure. The calibrated delivery pressure maythen be substituted for the sensed delivery pressure in the feedbackcontrol performed by the controller, which may advantageously improvethe accuracy of the lift pump control and thereby improve fuel economy.This exemplary calibration strategy is illustrated in FIG. 12B, whichwill be discussed below, and is appropriate when the pressure sensor isconsistently reading low. However, other methods of calibration of thesensed delivery pressure may be performed, without departing from thescope of this disclosure.

After 1108, the routine proceeds to 1110. Further, if the answer at 1104is NO, indicating that the sensed delivery pressure has reached thedesired peak delivery pressure, the routine proceeds to 1110. At 1110,the routine includes turning the lift pump OFF. For example, asdiscussed above with reference to routine 530, this may include thecontroller adjusting an actuator of the lift pump to apply apredetermined lower voltage level to the lift pump which is low enoughto keep the lift pump energized (e.g., greater than 0 V and less than0.3 V) and which does not substantially increase fuel pressure. In otherexamples, however, turning the lift pump OFF may include the controlleradjusting an actuator of the lift pump to apply 0 V to the lift pump.The predetermined lower voltage level may be determined via execution ofroutine 500, for example. By turning the lift pump OFF when the ONduration of the lift pump has reached the calibrated maximum duration,regardless of whether the sensed delivery pressure has reached the peakdelivery pressure, pulsed operation of the lift pump may continue evenwhen the pressure sensor is degraded. For example, if the sensor isdegraded and reading low, the sensed delivery pressure may remain flatat a level which is below the desired peak pressure when normalclosed-loop control of the lift pump (e.g., the routine of FIG. 5C) isperformed, as the control strategy may not turn the lift pump OFF untilthe desired peak pressure has been reached. In contrast, the robustcontrol strategy of routine 1100 “resets” the lift pump control when thelift pump has reached the calibrated maximum ON time, regardless ofwhether the delivery pressure has reached the desired peak deliverypressure.

After 1110, the routine proceeds to 1112 and the controller determinesan intended pressure drop ΔP between the peak pressure and the troughpressure, as well as a system stiffness S. The intended pressure drop ΔPrepresents the desired extent to which the delivery pressure decreasesduring the time period starting when the lift pump is turned OFF andending when the lift pump is turned ON again, and may be equal to thedifference between the desired peak pressure and the desired troughpressure, for example. System stiffness S may represent the bulk modulusof the fluid within the fuel system (e.g., fuel, or fuel and air). Thebulk modulus may be a function of the density of the fluid within thefuel system, and may be represented by the equation

${= {\rho\frac{d\; P}{d\;\rho}}},$where ρ is the density of the fluid in the fuel system, P is thepressure in the fuel system (e.g., the delivery pressure). The value ofS may be obtained at the controller via a lookup table stored in memoryat the controller, or alternatively may be calculated at the controlleras a function of currently sensed parameter values such as senseddelivery pressure as well as known dimensions of the fuel system (e.g.,the volume of a fuel passage within the fuel system) stored in memory atthe controller. Notably, as the equation relies on the rate of change ofsensed delivery pressure, as opposed to the magnitude of the senseddelivery pressure, it may be possibly to accurately determine S evenwhen the pressure sensor output is offset due to degradation.

After 1112, the routine proceeds to 1114 and the controller determines avolume V of fuel ingested by the engine while the lift pump is OFF whichshould trigger a transition in the state of the lift pump from OFF toON. V may be determined at the controller as a function of ΔP and S, forexample via the equation

${= \frac{\Delta\; P}{s}},$in one non-limiting example. The determined volume V represents thevolume of fuel which, when consumed by the engine (e.g., via fuelinjection) starting when the delivery pressure is at the desired peakpressure, should reduce the delivery pressure from the desired peakpressure to the desired trough pressure, given the current stiffness Sof the fuel system. If the volume V of fuel has been ingested by theengine since the lift pump was turned OFF with the delivery pressure atthe desired peak pressure, and yet the sensed delivery pressure is stillgreater than the desired trough pressure, this may indicate that thepressure sensor is degraded (e.g., reading high) or the fuel vaporpressure is higher than the value stored at the controller.

After 1114, the routine proceeds to 1116 and the controller determineswhether the sensed delivery pressure is greater than the desired troughdelivery pressure. If the answer at 1116 is NO, indicating that thesensed delivery pressure has reached the desired trough pressure, theroutine returns to 1102 to turn the lift pump ON, and another voltagepulse is applied to the lift pump. However, it will be appreciated thatroutine 1100 may be interrupted at any time (e.g., via a systeminterrupt) to end the robust feedback control of the lift pump.

Otherwise, if the answer at 1116 is YES, the routine proceeds to 1118and the controller determines whether the volume of fuel ingested by theengine since the lift pump was turned OFF is greater than the volume Vdetermined at 1114. The volume of fuel ingested by the engine since thelift pump was turned OFF may be equal to the amount of fuel injected tothe engine by the fuel system during the time period starting when thelift pump was turned OFF and ending upon execution of 1118, and may bedetermined at the controller as a function of sensed values and/or datastored in memory regarding the control of the fuel injectors during thetime period in question.

If the answer at 1118 is NO, the routine returns to 1116. Otherwise, ifthe answer at 1118 is YES, the routine returns to 1102, or optionallyproceeds to 1120 before returning to 1102.

At 1120, the controller calibrates the output of the pressure sensorsensing delivery pressure, so as to produce a more accurate indicationof the actual delivery pressure. When the volume of fuel ingested by theengine since the lift pump was turned OFF is greater than the volume V,this may occur due to flattening of the signal from the sensor whichreflects the actual delivery pressure being equal to the fuel vaporpressure. Such flattening may be determined in accordance with themethod of FIG. 7, for example. In one exemplary calibration strategy,upon determination that the volume of fuel ingested by the engine sincethe lift pump was turned OFF is greater than the volume V, thecontroller proceeds to determine whether the sensed delivery pressurehas remained constant for more than a threshold duration (e.g.,flattened). If so, going forward, the controller determines a pressureoffset as the difference between the pressure at which the senseddelivery pressure flattened and the fuel vapor pressure, and calibratesthe output of the pressure sensor by subtracting the offset from thesensed delivery pressure. Thus, the calibrated delivery pressuregenerated at the controller at a given time may be equal to thecurrently sensed delivery pressure minus the offset. The calibrateddelivery pressure may then be substituted for the sensed deliverypressure in the feedback control performed by the controller, which mayadvantageously improve engine operation by reducing the possibility of astall due to low fuel rail pressure. This exemplary calibration strategyis illustrated in FIG. 12D, which will be discussed below, and isappropriate when the pressure sensor is consistently reading high.

After 1120, the routine returns to 1102, or optionally ends if thecontroller terminates robust feedback control of the lift pump, e.g. dueto engine shutdown. By returning to 1102 and turning the lift pump ONwhen the volume of fuel consumed by the engine reaches a specifiedlevel, regardless of whether the sensed delivery pressure has decreasedto the desired trough delivery pressure, pulsed operation of the liftpump may continue even when the pressure sensor is degraded. Forexample, if the sensor is degraded and reading high, the sensed deliverypressure may remain flat at a level which is above the desired troughpressure when normal closed-loop control of the lift pump (e.g., theroutine of FIG. 5C) is performed, as the control strategy may not turnthe lift pump ON until the desired trough pressure has been reached. Incontrast, the robust control strategy of routine 1100 “resets” the liftpump control when a certain amount of fuel has been ingested by theengine, regardless of whether the delivery pressure has reached thedesired trough delivery pressure. Such control may advantageously reducestalling of the engine due to insufficient fuel delivery pressure.

It will be appreciated that if calibration of the sensed deliverypressure is initiated during execution of routine 1100 during a givenoperating period, the calibrated delivery pressure may be substitutedfor the sensed delivery pressure in subsequent iterations of routine1100 during that operating period. Depending on the accuracy of thecalibrated delivery pressure, further calibration may not be neededduring subsequent execution of routine 1100. Alternatively, ifdegradation of the pressure sensor escalates, further calibration may beperformed.

FIGS. 12A-12D show an example maps illustrating signals of interestduring control of the lift pump in accordance with the third exemplaryfeedback control strategy, e.g. in accordance with routine 1100 of FIG.11. For the sake of simplicity, in the depicted maps, the engine isoperating at steady state, fuel is ingested by the engine at a constantrate, and the magnitude of each voltage pulse applied to the lift pumpis the same.

Turning first to FIG. 12A, it shows an example map 1200 which depictsthe voltage applied to the lift pump at plot 1202, the actual deliverypressure of the lift pump at plot 1204, and the sensed delivery pressureof the lift pump at plot 1206. For all of these plots, the X-axisrepresents time, with time increasing along the X-axis from left toright. The Y-axis of each individual plot corresponds to the labeledparameter, with the value increasing from bottom to top. Additionally,line 1208 represents the (actual) pressure relief valve setpointpressure, line 1210 represents the desired peak delivery pressure, line1212 represents the desired trough delivery pressure, and line 1214represents the (actual) fuel vapor pressure.

Shortly after t0, the sensed delivery pressure reaches the desiredtrough delivery pressure 1212; in response, the controller turns thelift pump ON (e.g., by sending a signal to an actuator of the liftpump). However, the pressure sensor sensing the delivery pressure isreading low; the sensed delivery pressure is lower than the actualdelivery pressure by a first amount. Accordingly, the lift pump isturned ON when the actual delivery pressure is higher than the desiredtrough delivery pressure. Here, the first amount happens to be smallerthan the difference between the pressure relief valve setpoint pressureand the desired peak pressure. As such, when the sensed deliverypressure reaches the desired peak pressure, the actual delivery pressurehas not yet reached the pressure relief valve setpoint pressure. Inresponse to detecting that the sensed delivery pressure has reached thedesired peak pressure, the controller turns the lift pump OFF. Prior tobeing turned OFF, the lift pump was ON for an ON time 1216, which isless than a calibrated maximum ON time 1218 of the lift pump. After thelift pump is turned off, the actual delivery pressure decreases at arate which corresponds to the rate at which fuel from the fuel system isingested by the engine.

At t1, the pressure sensor degrades further and begins reading evenlower, such that the sensed delivery pressure is lower than the actualdelivery pressure by a second amount, the second amount greater than thefirst amount. The second amount happens to be larger than the differencebetween the pressure relief valve setpoint pressure and the desired peakpressure. The sensed delivery pressure decreases to the desired troughpressure at t2, before the actual delivery pressure has decreased to thedesired trough pressure (as the pressure sensor is reading low). Hereagain, the controller turns the lift pump ON at t2 upon detecting thatthe sensed delivery pressure has reached the desired trough pressure.

At t3, the actual delivery pressure reaches the pressure relief valvesetpoint pressure, which causes the pressure relief valve to open andbleed off excess fuel pressure. However, because the difference betweenthe actual delivery pressure and the sensed delivery pressure is greaterthan the difference between the pressure relief valve setpoint pressureand the desired peak delivery pressure, the sensed delivery pressure hasnot yet reached the desired peak pressure at t2. Accordingly, the liftpump remains ON, the actual delivery pressure remains constant(flattens) at the pressure relief valve setpoint pressure, and thesensed delivery pressure remains constant (flattens) at a pressure whichis below the desired peak pressure.

At t4, the controller detects that the lift pump has remained ON for thecalibrated maximum ON time 1218, and in response turns the lift pumpOFF, as discussed above with regard to routine 1100. Accordingly, eventhough the sensed delivery pressure has not reached the desired peakpressure, the length of time that the lift pump has been ON withoutreaching the desired peak pressure indicates that the output of thesensor may be inaccurate, and the controller turns the lift pump OFF sothat pulsed operation may be continued. Such operation is in contrast tothe second exemplary feedback control strategy discussed herein, inwhich the lift pump is only turned OFF when the sensed delivery pressurereaches the desired peak pressure, which can result in the lift pumpcontinuing to be ON even though the actual delivery pressure hasexceeded the desired peak pressure and reached the pressure relief valvesetpoint pressure.

After t4, the controller continues to turn the lift pump ON when thesensed delivery pressure has decreased to the desired trough pressure,and turn the lift pump OFF when the calibrated maximum ON time has beenreached. As shown, because the extent to which the sensor reads lowremains constant after t4, the lift pump remains ON for the calibratedmaximum ON time 1218 each time a voltage pulse is applied. Thus,although the sensor is degraded and reading low, the robust feedbackcontrol strategy enables pulsed operation of the lift pump to beperformed, thereby improving fuel economy.

FIG. 12B shows an example map 1240 which illustrates the same signals asmap 1200, and also represents lift pump operation in accordance with thethird exemplary feedback control strategy. However, in map 1240, thecontroller initiates calibration of the output of the pressure sensorupon detection that the lift pump has remained ON for the calibratedmaximum ON time at t3. Plot 1242 represents the calibrated pressuresensor output.

In the depicted example, the controller determines the calibratedpressure sensor output by adding an offset 1244 to the sensed deliverypressure which is equal to the difference between the pressure reliefvalve setpoint pressure and the pressure at which the sensed deliverypressure flattened between t2 and t3. From t3 onward, the feedbackcontrol is performed based on the calibrated pressure sensor output1242, rather than the sensed delivery pressure 1206. Accordingly, whenthe sensed delivery pressure reaches the desired trough pressure at t4,the controller does not turn the lift pump ON; instead, the lift pumpremains OFF until the calibrated pressure sensor output reaches thedesired trough pressure at t5. Similarly, once the calibrated pressuresensor output reaches the desired peak pressure at t6, the lift pump isturned OFF, even though the sensed delivery pressure has not yet reachedthe desired peak pressure. As shown, the calibrated pressure sensoroutput 1242 closely matches the actual delivery pressure 1204 from t3onward, such that the lift pump may be controlled accurately andefficiently despite the erroneous output of the pressure sensor.

FIG. 12C shows an example map 1260 which illustrates the same signals asmap 1200, and also represents lift pump operation in accordance with thethird exemplary feedback control strategy. However, whereas maps 1200and 1240 illustrate lift pump operation during sensor degradation whichcauses the sensor to read low, map 1260 illustrates lift pump operationduring sensor degradation which causes the sensor to read high. Map 1260additionally illustrates the volume of fuel ingested by the engine atplot 1262, and provides exemplary values for the actual deliverypressure and sensed delivery pressure. Specifically, in the depictednon-limiting example, the desired trough delivery pressure is 400 kPa,and the desired peak delivery pressure is 600 kPa.

Shortly after t0, the sensed delivery pressure has decreased to thedesired trough pressure, and thus the controller turns the lift pump ON.At this time, because the sensor is reading high, the actual deliverypressure is less than the desired trough pressure by a first amount.Here, the first amount happens to be smaller than the difference betweenthe desired trough pressure and the fuel vapor pressure. As such, whenthe sensed delivery pressure reaches the desired trough pressure, theactual delivery pressure has not yet reached the fuel vapor pressure,and thus the signals have not flattened. After the lift pump is turnedON, the actual delivery pressure decreases at a rate which correspondsto the magnitude of voltage applied to the lift pump.

At t1, the sensed delivery pressure reaches the desired peak pressure,and in response the controller turns the lift pump OFF. Because thepressure sensor is reading high, the actual delivery pressure has notyet reached the desired peak pressure. Accordingly, the deliverypressure is lower than the requested delivery pressure for currentengine operation.

At t2, the pressure sensor degrades further and begins reading evenhigher, such that the sensed delivery pressure is higher than the actualdelivery pressure by a second amount, the second amount greater than thefirst amount. The second amount happens to be larger than the differencebetween the desired trough pressure and the fuel vapor pressure. Theactual delivery pressure decreases to the desired trough pressureslightly before t2, and then reaches the fuel vapor pressure at t2 whichcauses the signal to flatten. Because the pressure sensor is readinghigh, and because the second amount is larger than the differencebetween the desired trough pressure and the fuel vapor pressure, thesensed delivery pressure flattens at a pressure higher than the desiredtrough pressure. Because the sensed delivery pressure has not reachedthe desired trough pressure, the controller does not turn the lift pumpON, and the actual delivery pressure remains at the fuel vapor pressure.If this were to continue for too long, the engine could stall.

In order to prevent a stall, as discussed above with reference toroutine 1100, the controller monitors the volume of fuel ingested by theengine and compares it with a volume V of fuel ingested by the enginewhile the lift pump is OFF which should trigger a transition in thestate of the lift pump from OFF to ON. As noted above, volume V may beequal to the quotient of the intended pressure drop ΔP between thedesired peak pressure and the desired trough pressure and a systemstiffness S. In the depicted example, the intended pressure drop ΔP is200 kPa, and the system stiffness S is 100 kPa/cc, and thus volume V is2 cc. For the sake of example, plot 1262 indicates that 2 cc of fuel hasbeen ingested at t1, 4 cc of fuel has been ingested at t4, and so on;this is simply illustrative, and does not represent actual cumulativequantities of fuel ingested that would occur over the course of engineoperation. In other examples, the controller may reset the volume offuel ingested V to 0 every time the lift pump is turned OFF.

At t1, when the lift pump was turned OFF, the volume of fuel ingestedwas at 2 cc. At t4, the volume of fuel ingested has reached 4 cc, andthus 2 cc of fuel has been ingested since the lift pump was turned OFF.Since volume V to trigger a transition in the state of the lift pump isset to 2 cc, the controller turns ON the lift pump at t4. Accordingly,even though the sensed delivery pressure has not reached the desiredtrough pressure, the lift pump is turned ON once the volume V has beeningested, so that pulsed operation may be continued. Such operation isin contrast to the second exemplary feedback control strategy discussedherein, in which the lift pump is only turned back ON when the senseddelivery pressure reaches the desired trough pressure, which can resultin the lift pump continuing to be OFF even though the actual deliverypressure has reached the fuel vapor pressure.

After t4, the controller continues to turn the lift pump OFF when thesensed delivery pressure has increased to the desired peak pressure, andturn the lift pump ON when the volume of fuel ingested by the enginesince the lift pump was turned OFF reaches 2 cc. Thus, although thesensor is degraded and reading high, the robust feedback controlstrategy enables pulsed operation of the lift pump to be performed,thereby improving fuel economy.

FIG. 12D shows an example map 1280 which illustrates the same signals asmap 1260, and also represents lift pump operation in accordance with thethird exemplary feedback control strategy. However, in map 1280, thecontroller initiates calibration of the output of the pressure sensorupon detection that volume V of fuel has been ingested since the liftpump was turned OFF. Plot 1282 represents the calibrated pressure sensoroutput.

In the depicted example, the controller determines the calibratedpressure sensor output by subtracting an offset 1284 from the senseddelivery pressure which is equal to the difference between the pressureat which the sensed delivery pressure flattened between t3 and t4 andthe fuel vapor pressure. From t4 onward, the feedback control isperformed based on the calibrated pressure sensor output 1282, ratherthan the sensed delivery pressure 1206. Accordingly, when the senseddelivery pressure reaches the desired peak pressure at t5, thecontroller does not turn the lift pump ON; instead, the lift pumpremains OFF until the calibrated pressure sensor output reaches thedesired peak pressure at t6. Similarly, once the calibrated pressuresensor output reaches the desired trough pressure at t7, the lift pumpis turned OFF, even though the sensed delivery pressure has not yetreached the desired trough pressure. As shown, the calibrated pressuresensor output 1282 closely matches the actual delivery pressure 1204from t4 onward, such that the lift pump may be controlled accurately andefficiently despite the erroneous output of the pressure sensor.

In accordance with the above description, a method for an engineincludes, during pulsed mode operation of a lift pump, adjusting a levelof voltage applied to the lift pump based on an output signal of apressure sensor downstream of the lift pump and monitoring the outputsignal for flattening; and in response to a detection of flattening,indicating a pressure sensor error and operating the lift pumpindependent of the output signal of the pressure sensor. In a firstexample of the method, monitoring the output signal for flatteningcomprises comparing a duration of time during which a slope of theoutput signal is zero to a threshold duration. A second example of themethod optionally includes the first example and further includeswherein operating the lift pump independent of the output signal of thepressure sensor comprises operating the lift pump in a continuous modein which a constant non-zero voltage is applied to the lift pump. Athird example of the method optionally includes one or more of the firstand second examples, and further includes wherein operating the liftpump independent of the output signal of the pressure sensor comprisesoperating the lift pump in a pulsed mode in which the level of voltageapplied to the lift pump is not adjusted based on the output signal ofthe pressure sensor. A fourth example of the method optionally includesone or more of the first through third examples, and further includeswherein adjusting the level of voltage applied to the lift pump based onthe output signal of the pressure sensor comprises adjusting a dutycycle of the voltage pulses based on the output signal. A fifth exampleof the method optionally includes one or more of the first throughfourth examples, and further includes wherein adjusting the duty cycleof the voltage pulses based on the output signal comprises increasingthe duty cycle when a peak pressure of the output signal is less than adesired peak pressure, and decreasing the duty cycle when the peakpressure is greater than the desired peak pressure. A sixth example ofthe method optionally includes one or more of the first through fifthexamples, and further includes wherein adjusting the level of voltageapplied to the lift pump based on the output signal of the pressuresensor comprises applying a first, higher voltage to the lift pump whenthe output signal of the pressure sensor decreases to a desired troughpressure and applying a second, lower voltage to the lift pump when theoutput signal of the pressure sensor increases to a desired peakpressure. A seventh example of the method optionally includes one ormore of the first through sixth examples, and further includes, whereinthe pressure sensor error is an in-range error, the method furthercomprising, in response to the output signal increasing above ordecreasing below an expected operating range of the pressure sensor,indicating an out-of-range error of the pressure sensor and operatingthe lift pump independent of the output signal of the pressure sensor.

Further, in accordance with the above description, an additional methodfor operating an engine fuel system comprises during steady state engineoperation with a requested delivery pressure of a fuel lift pump below afirst threshold, decreasing a duty cycle of voltage pulses applied to afuel lift pump until flattening of an output signal of a pressure sensordownstream of the lift pump is detected, and storing the pressure atwhich the output signal flattened as a fuel vapor pressure of the fuelsystem; during steady state engine operation with a requested deliverypressure of the fuel lift pump above a second threshold, increasing aduty cycle of voltage pulses applied to the lift pump until flatteningof the output signal of the pressure sensor is detected, storing thepressure at which the output signal flattened as a setpoint pressure ofa pressure relief valve; and adjusting lift pump operation based on thestored setpoint pressure and fuel vapor pressure.

In a first example of the additional method, adjusting lift pumpoperation based on the stored setpoint pressure and fuel vapor pressurecomprises adjusting a desired peak delivery pressure of the lift pump tobe less than the stored setpoint pressure by a first predeterminedamount and adjusting a desired trough pressure of the lift pump to begreater than the stored fuel vapor pressure by a second predeterminedamount. A second example of the additional method optionally includesthe first example and further includes wherein adjusting operation ofthe lift pump based on the stored setpoint pressure and fuel vaporpressure further comprises, during operation of the lift pump in apulsed mode, applying a first, higher voltage to the lift pump everytime the output signal of the pressure sensor decreases to the desiredtrough pressure and applying a second, lower voltage to the lift pumpevery time the output signal of the pressure sensor increases to thedesired peak pressure. A third example of the additional methodoptionally includes one or more of the first and second examples, andfurther includes wherein adjusting lift pump operation based on thestored setpoint pressure and fuel vapor pressure comprises determining aduty cycle of voltage pulses which, when applied to the lift pump, willproduce an output signal having a maximum value at the desired peakdelivery pressure and a minimum value at the desired trough deliverypressure, and applying voltage pulses to the lift pump with thedetermined duty cycle. A fourth example of the additional methodoptionally includes one or more of the first through third examples, andfurther includes wherein the requested delivery pressure of the fuellift pump is directly proportional to engine load. A fifth example ofthe additional method optionally includes one or more of the firstthrough fourth examples, and further includes, while applying voltagepulses to the lift pump with the determined duty cycle, monitoring theoutput signal of the pressure sensor for flattening, and in response toa detection of flattening, indicating a pressure sensor error andoperating the lift pump independent of the output signal of the pressuresensor. A sixth example of the additional method optionally includes oneor more of the first through fifth examples, and further includeswherein operating the lift pump independent of the output signal of thepressure sensor comprises operating the lift pump in a continuous modein which a constant non-zero voltage is applied to the lift pump oroperating the lift pump in a pulsed mode in which the voltage pulsesapplied to the lift pump are not adjusted based on the output signal ofthe pressure sensor.

Furthermore, in accordance with the above description, a hybrid vehiclecomprises a powertrain comprising an engine, a motor/generator, abattery, and a transmission coupled to vehicle wheels; a fuel systemcomprising a fuel tank, a fuel lift pump, a pressure sensor arrangeddownstream of an output of the lift pump in the fuel system, and apressure relief valve; a controller including non-transitory memory withinstructions stored therein which are executable by a processor to: inresponse to a request to dynamically learn a fuel vapor pressure of thefuel system during pulsed operation of the lift pump with requestedvehicle wheel torque above a first threshold, mechanically couple acrankshaft of the engine to the motor/generator, decrease engine loaduntil an output signal of the pressure sensor remains constant for atleast a first threshold duration while converting electrical energy totorque with the motor/generator and providing the torque the vehiclewheels, and store the pressure at which the output signal remainsconstant as the fuel vapor pressure. In a first example of the hybridvehicle, the controller further comprises instructions stored innon-transitory memory and executable by a processor to: in response to arequest to dynamically learn a setpoint pressure of the pressure reliefvalve during pulsed operation of the lift pump with requested engineoutput torque below a second threshold, mechanically couple thecrankshaft to the motor/generator, increase engine load until the outputsignal of the pressure sensor remains constant for at least a secondthreshold duration while converting a portion of engine output torque toelectrical energy with the motor/generator and storing the electricalenergy at the battery, and store the pressure at which the output signalremains constant as the setpoint pressure. A second example of thehybrid vehicle optionally includes the first example and furtherincludes wherein the controller further comprises instructions stored innon-transitory memory and executable by a processor to: while performingclosed-loop control of the lift pump based on an output signal of thepressure sensor, monitor the output signal; in response to the outputsignal remaining constant for at least a threshold duration, indicate anin-range error of the pressure sensor and switch from closed-loop toopen-loop control of the lift pump in which lift pump operation isadjusted independent of the output signal of the pressure sensor. Athird example of the hybrid vehicle optionally includes one or more ofthe first and second examples, and further includes wherein theinstructions stored in non-transitory memory and executable by theprocessor to switch from closed-loop to open-loop control of the liftpump in which lift pump operation is adjusted independent of the outputsignal of the pressure sensor comprise instructions to apply acontinuous non-zero voltage to the lift pump. A fourth example of thehybrid vehicle optionally includes one or more of the first throughthird examples, and further includes wherein the controller furthercomprises instructions stored in non-transitory memory and executable bya processor to, after storing the pressure at which the output signalremains constant as the fuel vapor pressure, adjust a duty cycle ofvoltage pulses applied to the lift pump based on a desired pressuremargin between the fuel vapor pressure and lift pump delivery pressure.

Moreover, in accordance with the above description, a method ofoperating an engine fuel system, comprises during pulsed operation of alift pump, turning the lift pump OFF when a sensed delivery pressureincreases to a desired peak pressure or an ON time of the lift pumpreaches a calibrated maximum and turning the lift pump ON when eitherthe sensed delivery pressure decreases to a desired trough pressure or avolume of fuel ingested by the engine reaches a predetermined volume. Afirst example of this method includes determining the predeterminedvolume as a function of a difference between the desired peak pressureand the desired trough pressure and a stiffness of the fuel system. Asecond example of this method optionally includes the first example andfurther includes wherein the predetermined volume is set equal to thequotient of the difference between the desired peak pressure and thedesired trough pressure and the stiffness of the fuel system. A thirdexample of this method optionally includes one or more of the first andsecond examples, and further includes determining the stiffness of thefuel system as a function of a density of fluid within the fuel system.A fourth example of this method optionally includes one or more of thefirst through third examples, and further includes, in response to theON time of the lift pump reaching the calibrated maximum, indicating anin-range error of the pressure sensor and initiating calibration of thesensed delivery pressure, the calibration including adding an offset tothe sensed delivery pressure. A fifth example of this method optionallyincludes one or more of the first through fourth examples, and furtherincludes wherein the offset is equal to the difference between asetpoint pressure of a pressure relief valve and the sensed deliverypressure when the ON time reached the calibrated maximum. A sixthexample of this method optionally includes one or more of the firstthrough fifth examples, and further includes, in response to the volumeof fuel ingested by the engine reaching the predetermined volume,indicating an in-range error of the pressure sensor and initiatingcalibration of the sensed delivery pressure, the calibration includingsubtracting an offset from the sensed delivery pressure. A seventhexample of this method optionally includes one or more of the firstthrough sixth examples, and further includes wherein the offset is equalto the difference between the sensed delivery pressure when the volumeof fuel ingested by the engine reached the predetermined volume and afuel vapor pressure of the fuel system.

Yet another method in accordance with the present disclosure includes,while performing closed-loop control of a lift pump based on an outputsignal of a pressure sensor arranged downstream of the lift pump,monitoring the output signal; in response to the output signal remainingconstant for at least a first threshold duration while the lift pump isON, turning the lift pump OFF, calibrating the output signal based onthe pressure at which the output signal remained constant, andperforming subsequent closed-loop control of the lift pump based on thecalibrated output signal; in response to the output signal remainingconstant for at least a second threshold duration while the lift pump isOFF, turning the lift pump ON, calibrating the output signal based onthe pressure at which the output signal remained constant, andperforming subsequent closed-loop control of the lift pump based on thecalibrated output signal. A first example of this method includeswherein calibrating the output signal based on the pressure at which theoutput signal remained constant while the lift pump was ON comprisesadding a first offset to the output signal, the first offset equal tothe difference between a setpoint pressure of a pressure relief valveand the pressure at which the output signal remained constant while thelift pump was ON. A second example of this method optionally includesthe first example and further includes wherein calibrating the outputsignal based on the pressure at which the output signal remainedconstant while the lift pump was OFF comprises subtracting a secondoffset from the output signal, the second offset equal to the differencebetween the pressure at which the output signal remained constant whilethe lift pump was OFF and a fuel vapor pressure of the fuel system. Athird example of this method optionally includes one or more of thefirst and second examples, and further includes determining the firstthreshold duration by subtracting an ON time of the lift pump prior tothe output signal reaching the pressure at which it remained constantfrom a calibrated maximum ON time. A fourth example of this methodoptionally includes one or more of the first through third examples, andfurther includes determining the second threshold duration based on acurrent rate of fuel ingestion by the engine and a difference between apredetermined volume of fuel and a volume of fuel ingested by the liftpump since the lift pump was turned OFF prior to the output signalreaching the pressure at which it remained constant. A fifth example ofthis method optionally includes one or more of the first through fourthexamples, and further includes wherein the predetermined volume of fuelis determined as a function of a difference between a desired peakdelivery pressure and a desired trough delivery pressure and a stiffnessof the fuel system. A sixth example of this method optionally includesone or more of the first through fifth examples, and further includeswherein the predetermined volume is set equal to the quotient of thedifference between the desired peak pressure and the desired troughpressure and the stiffness of the fuel system, and where the stiffnessof the fuel system is determined as a function of a density of fluidwithin the fuel system.

Additionally, in accordance with the above description, a hybrid vehiclecomprises a powertrain comprising an engine, a motor/generator, abattery, and a transmission coupled to vehicle wheels; a fuel systemcomprising a fuel tank, a fuel lift pump, a pressure sensor arrangeddownstream of an output of the lift pump in the fuel system, and apressure relief valve; and a controller including non-transitory memorywith instructions stored therein which are executable by a processor to:during pulsed operation of a lift pump, monitor a volume of fuelingested by the engine while the lift pump is OFF; if the volume of fuelingested by the engine while the lift pump is OFF reaches apredetermined volume before an output signal of the pressure sensor hasdecreased to a desired trough pressure, turn the lift pump ON, store thevalue of the output signal of the pressure sensor as a first storedvalue, and request dynamic learning of a fuel vapor pressure of the fuelsystem; if a requested vehicle wheel torque is above a first threshold,mechanically couple a crankshaft of the engine to the motor/generator,decrease engine load until the output signal of the pressure sensorremains constant for at least a first threshold duration whileconverting electrical energy to torque with the motor/generator andproviding the torque the vehicle wheels, and store the pressure at whichthe output signal remains constant as an updated fuel vapor pressure;and if the updated fuel vapor pressure is less than the first storedvalue, indicate that the pressure sensor is reading high. In a firstexample of the hybrid vehicle, the controller further comprisesinstructions stored in non-transitory memory and executable by aprocessor to: during pulsed operation of the lift pump, monitor an ONtime of the lift pump; if the ON time of the lift pump reaches acalibrated maximum ON time before the output signal of the pressuresensor has increased to a desired peak pressure, turn the lift pump OFF,store the value of the output signal of the pressure sensor as a secondstored value, and request dynamic learning of a setpoint pressure of thepressure relief valve; if the requested engine output torque is below asecond threshold, mechanically couple the crankshaft to themotor/generator, increase engine load until the output signal of thepressure sensor remains constant for at least a second thresholdduration while converting a portion of engine output torque toelectrical energy with the motor/generator and storing the electricalenergy at the battery, and store the pressure at which the output signalremains constant as an updated setpoint pressure; and if the updatedsetpoint pressure is greater than the second stored value, indicate thatthe pressure sensor is reading low. A second example of the hybridvehicle optionally includes the first example and further includeswherein the controller further comprises instructions stored innon-transitory memory and executable by a processor to: in response toan indication that the pressure sensor is reading high, initiatingcalibration of the output signal of the pressure sensor, the calibrationincluding subtracting a first offset from the output signal of thepressure sensor. A third example of the hybrid vehicle optionallyincludes one or more of the first and second examples, and furtherincludes wherein the controller further comprises instructions stored innon-transitory memory and executable by a processor to: in response toan indication that the pressure sensor is reading low, initiatingcalibration of the output signal of the pressure sensor, the calibrationincluding adding a second offset to the output signal of the pressuresensor. A fourth example of the hybrid vehicle optionally includes oneor more of the first through third examples, and further includeswherein the controller further comprises instructions stored innon-transitory memory and executable by a processor to set the firstoffset equal to the difference between the first stored value and theupdated fuel vapor pressure, and to set the second offset equal to thedifference between the updated setpoint pressure and the second storedvalue.

In accordance with the methods and systems disclosed herein, in-rangeerrors of a pressure sensor measuring lift pump delivery pressure may bedetected accurately. In response to detection of an in-range error ofthe pressure sensor, lift pump control may be switched from aclosed-loop control strategy, in which a duty cycle of voltage pulsesapplied to the lift pump is adjusted based on feedback from the pressuresensor, to an open-loop control strategy, in which the voltage appliedto the lift pump is independent of feedback from the pressure sensor.Notably, the detection of in-range errors may include detection offlattening of sensed pressure without consideration of the magnitude ofthe sensed pressure, which has the technical effect of identifyingdegradation of the pressure sensor even when the pressure sensor isoperating within its expected operating range, and which mayadvantageously reduce control complexity. Further, switching fromclosed-loop control to open-loop control of the lift pump upon detectionof an in-range error may allow the fuel system to continue provided acommanded delivery pressure despite the malfunctioning of the pressuresensor. Alternatively, in accordance with the methods and systemsdisclosed herein, a robust closed-loop feedback control strategy may beperformed, which enables closed-loop pulsed operation of the lift pumpto continue even when flattening of the pressure sensor output hasindicated that the sensor is degraded.

In another representation, a method in accordance with the presentdisclosure may include, with a controller, adjusting operation of a fuellift pump of an engine fuel system to dynamically learn a setpointpressure of a pressure relief valve in the fuel system and a fuel vaporpressure of the fuel system; adjusting operation of the lift pump tomaintain a first desired margin between a maximum delivery pressure andthe setpoint pressure and a second desired margin between a minimumdelivery pressure and the fuel vapor pressure; and monitoring thedelivery pressure with a pressure sensor arranged downstream of the liftpump for a deviation from an expected slope of the sensed deliverypressure signal. The deviation may include the signal having a slope ofzero for longer than a predetermined threshold duration.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory and may be carried outby the control system including the controller in combination with thevarious sensors, actuators, and other engine hardware. The specificroutines described herein may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example embodiments described herein, butis provided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, thedescribed actions, operations and/or functions may graphically representcode to be programmed into non-transitory memory of the computerreadable storage medium in the engine control system, where thedescribed actions are carried out by executing the instructions in asystem including the various engine hardware components in combinationwith the electronic controller.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

The invention claimed is:
 1. A method of operating an engine fuelsystem, comprising: during pulsed mode operation of a lift pump,adjusting a level of voltage applied to the lift pump based on an outputsignal of a pressure sensor downstream of the lift pump and monitoringthe output signal for flattening; in response to a detection offlattening, indicating a pressure sensor error and operating the liftpump independent of the output signal of the pressure sensor.
 2. Themethod of claim 1, wherein monitoring the output signal for flatteningcomprises comparing a duration of time during which a slope of theoutput signal is zero to a threshold duration.
 3. The method of claim 2,wherein operating the lift pump independent of the output signal of thepressure sensor comprises operating the lift pump in a continuous modein which a constant non-zero voltage is applied to the lift pump.
 4. Themethod of claim 2, wherein operating the lift pump independent of theoutput signal of the pressure sensor comprises operating the lift pumpin a pulsed mode in which the level of voltage applied to the lift pumpis not adjusted based on the output signal of the pressure sensor. 5.The method of claim 1, wherein adjusting the level of voltage applied tothe lift pump based on the output signal of the pressure sensorcomprises adjusting a duty cycle of the voltage pulses based on theoutput signal.
 6. The method of claim 5, wherein adjusting the dutycycle of the voltage pulses based on the output signal comprisesincreasing the duty cycle when a peak pressure of the output signal isless than a desired peak pressure, and decreasing the duty cycle whenthe peak pressure is greater than the desired peak pressure.
 7. Themethod of claim 1, wherein adjusting the level of voltage applied to thelift pump based on the output signal of the pressure sensor comprisesapplying a first, higher voltage to the lift pump when the output signalof the pressure sensor decreases to a desired trough pressure andapplying a second, lower voltage to the lift pump when the output signalof the pressure sensor increases to a desired peak pressure.
 8. Themethod of claim 2, wherein the pressure sensor error is an in-rangeerror, the method further comprising, in response to the output signalincreasing above or decreasing below an expected operating range of thepressure sensor, indicating an out-of-range error of the pressure sensorand operating the lift pump independent of the output signal of thepressure sensor.
 9. A method for operating an engine fuel system,comprising: during steady state engine operation with a requesteddelivery pressure of a fuel lift pump below a first threshold,decreasing a duty cycle of voltage pulses applied to a fuel lift pumpuntil flattening of an output signal of a pressure sensor downstream ofthe lift pump is detected, storing the pressure at which the outputsignal flattened as a fuel vapor pressure of the fuel system; duringsteady state engine operation with a requested delivery pressure of thefuel lift pump above a second threshold, increasing a duty cycle ofvoltage pulses applied to the lift pump until flattening of the outputsignal of the pressure sensor is detected, and storing the pressure atwhich the output signal flattened as a setpoint pressure of a pressurerelief valve; and adjusting lift pump operation based on the storedsetpoint pressure and fuel vapor pressure.
 10. The method of claim 9,wherein adjusting lift pump operation based on the stored setpointpressure and fuel vapor pressure comprises adjusting a desired peakdelivery pressure of the lift pump to be less than the stored setpointpressure by a first predetermined amount and adjusting a desired troughpressure of the lift pump to be greater than the stored fuel vaporpressure by a second predetermined amount.
 11. The method of claim 10,wherein adjusting operation of the lift pump based on the storedsetpoint pressure and fuel vapor pressure further comprises, duringoperation of the lift pump in a pulsed mode, applying a first, highervoltage to the lift pump every time the output signal of the pressuresensor decreases to the desired trough pressure and applying a second,lower voltage to the lift pump every time the output signal of thepressure sensor increases to the desired peak pressure.
 12. The methodof claim 10, wherein adjusting lift pump operation based on the storedsetpoint pressure and fuel vapor pressure comprises determining a dutycycle of voltage pulses which, when applied to the lift pump, willproduce an output signal having a maximum value at the desired peakdelivery pressure and a minimum value at the desired trough deliverypressure, and applying voltage pulses to the lift pump with thedetermined duty cycle.
 13. The method of claim 9, wherein the requesteddelivery pressure of the fuel lift pump is directly proportional toengine load.
 14. The method of claim 12, further comprising, whileapplying voltage pulses to the lift pump with the determined duty cycle,monitoring the output signal of the pressure sensor for flattening, andin response to a detection of flattening, indicating a pressure sensorerror and operating the lift pump independent of the output signal ofthe pressure sensor.
 15. The method of claim 14, wherein operating thelift pump independent of the output signal of the pressure sensorcomprises operating the lift pump in a continuous mode in which aconstant non-zero voltage is applied to the lift pump or operating thelift pump in a pulsed mode in which the voltage pulses applied to thelift pump are not adjusted based on the output signal of the pressuresensor.
 16. A hybrid vehicle, comprising: a powertrain comprising anengine, a motor/generator, a battery, and a transmission coupled tovehicle wheels; a fuel system comprising a fuel tank, a fuel lift pump,a pressure sensor arranged downstream of an output of the lift pump inthe fuel system, and a pressure relief valve; a controller includingnon-transitory memory with instructions stored therein which areexecutable by a processor to: in response to a request to dynamicallylearn a fuel vapor pressure of the fuel system during pulsed operationof the lift pump with requested vehicle wheel torque above a firstthreshold, mechanically couple a crankshaft of the engine to themotor/generator, decrease engine load until an output signal of thepressure sensor remains constant for at least a first threshold durationwhile converting electrical energy to torque with the motor/generatorand providing the torque the vehicle wheels, and store the pressure atwhich the output signal remains constant as the fuel vapor pressure. 17.The hybrid vehicle of claim 16, wherein the controller further comprisesinstructions stored in non-transitory memory and executable by aprocessor to: in response to a request to dynamically learn a setpointpressure of the pressure relief valve during pulsed operation of thelift pump with requested engine output torque below a second threshold,mechanically couple the crankshaft to the motor/generator, increaseengine load until the output signal of the pressure sensor remainsconstant for at least a second threshold duration while converting aportion of engine output torque to electrical energy with themotor/generator and storing the electrical energy at the battery, andstore the pressure at which the output signal remains constant as thesetpoint pressure.
 18. The hybrid vehicle of claim 17, wherein thecontroller further comprises instructions stored in non-transitorymemory and executable by a processor to: while performing closed-loopcontrol of the lift pump based on an output signal of the pressuresensor, monitor the output signal; in response to the output signalremaining constant for at least a threshold duration, indicate anin-range error of the pressure sensor and switch from closed-loop toopen-loop control of the lift pump in which lift pump operation isadjusted independent of the output signal of the pressure sensor. 19.The hybrid vehicle of claim 18, wherein the instructions stored innon-transitory memory and executable by the processor to switch fromclosed-loop to open-loop control of the lift pump in which lift pumpoperation is adjusted independent of the output signal of the pressuresensor comprise instructions to apply a continuous non-zero voltage tothe lift pump.
 20. The hybrid vehicle of claim 16, wherein thecontroller further comprises instructions stored in non-transitorymemory and executable by a processor to, after storing the pressure atwhich the output signal remains constant as the fuel vapor pressure,adjust a duty cycle of voltage pulses applied to the lift pump based ona desired pressure margin between the fuel vapor pressure and lift pumpdelivery pressure.